[libc++] Add missing XFAIL to tests that need __atomic_* libcalls

FreeBSD did not provide the __atomic_* functions as part of the base
system until recently. They were added to libgcc_s in SVN revision r364753
(August 2020), so check for availability of 'non-lockfree-atomics' so that
these tests do not fail unexpectedly on older versions of FreeBSD.

This also removes the #ifndef __APPLE__ from atomic_helpers.h that was used
to work around lack of atomic runtime functions on older Apple platforms
and replaces it with XFAIL: !non-lockfree-atomics.

Reviewed By: #libc, ldionne
Differential Revision: https://reviews.llvm.org/D88818

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_compare_exchange_strong.pass.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: libcpp-has-no-threads
+// XFAIL: !non-lockfree-atomics
 //  ... assertion fails line 34
 
 // <atomic>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_compare_exchange_strong_explicit.pass.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: libcpp-has-no-threads
+// XFAIL: !non-lockfree-atomics
 //  ... assertion fails line 38
 
 // <atomic>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_compare_exchange_weak.pass.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: libcpp-has-no-threads
+// XFAIL: !non-lockfree-atomics
 //  ... assertion fails line 34
 
 // <atomic>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_compare_exchange_weak_explicit.pass.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: libcpp-has-no-threads
+// XFAIL: !non-lockfree-atomics
 //  ... assertion fails line 38
 
 // <atomic>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_exchange.pass.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: libcpp-has-no-threads
+// XFAIL: !non-lockfree-atomics
 //  ... fails assertion line 31
 
 // <atomic>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_exchange_explicit.pass.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: libcpp-has-no-threads
+// XFAIL: !non-lockfree-atomics
 //  ... assertion fails line 32
 
 // <atomic>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_helpers.h

@@ -106,13 +106,14 @@ struct TestEachAtomicType {
     void operator()() const {
         TestEachIntegralType<TestFunctor>()();
         TestFunctor<UserAtomicType>()();
-#ifndef __APPLE__
         /*
-            These aren't going to be lock-free,
-            so some libatomic.a is necessary.
+            Note: These aren't going to be lock-free,
+            so some libatomic.a is necessary. To handle
+            the case where the support functions are
+            missing, all tests that use this file should add:
+            XFAIL: !non-lockfree-atomics
         */
         TestFunctor<LargeUserAtomicType>()();
-#endif
 /*
     Enable these once we have P0528 
     

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_init.pass.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: libcpp-has-no-threads
+// XFAIL: !non-lockfree-atomics
 //  ... assertion fails line 36
 
 // <atomic>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_is_lock_free.pass.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: libcpp-has-no-threads
+// XFAIL: !is-lockfree-runtime-function
 
 // <atomic>
 

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_load.pass.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: libcpp-has-no-threads
+// XFAIL: !non-lockfree-atomics
 //  ... assertion fails line 35
 
 // <atomic>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_load_explicit.pass.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: libcpp-has-no-threads
+// XFAIL: !non-lockfree-atomics
 //  ... assertion fails line 31
 
 // <atomic>

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_store.pass.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: libcpp-has-no-threads
+// XFAIL: !non-lockfree-atomics
 
 // <atomic>
 

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.req/atomic_store_explicit.pass.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 //
 // UNSUPPORTED: libcpp-has-no-threads
+// XFAIL: !non-lockfree-atomics
 
 // <atomic>
 

libcxx/test/std/atomics/atomics.types.operations/atomics.types.operations.wait/atomic_wait.pass.cpp

@@ -8,6 +8,7 @@
 //
 // UNSUPPORTED: libcpp-has-no-threads
 // XFAIL: c++03
+// XFAIL: !non-lockfree-atomics
 
 // This test requires the dylib support introduced in D68480,
 // which hasn't shipped yet.