[ubsan-minimal] Add a path for non-POSIX (and bare-metal) use of the library

Summary:
Hook on -DKERNEL_USE (which is also used in lib/builtins) to not import
strlen and not rely on write() being implemented with the stderr on fd 2.

With this, the only requirements to use this library are:
  - "Good enough" std::atomic<void*> and std::atomic<int>
  - abort() being implemented
  - ubsan_message(const char*) being implemented

Reviewers: eugenis

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39791

llvm-svn: 320831

compiler-rt/lib/ubsan_minimal/ubsan_minimal_handlers.cc

@@ -5,9 +5,14 @@
 #include <string.h>
 #include <unistd.h>
 
+#ifdef KERNEL_USE
+extern "C" void ubsan_message(const char *msg);
+static void message(const char *msg) { ubsan_message(msg); }
+#else
 static void message(const char *msg) {
   write(2, msg, strlen(msg));
 }
+#endif
 
 static const int kMaxCallerPcs = 20;
 static __sanitizer::atomic_uintptr_t caller_pcs[kMaxCallerPcs];