unwind: allow building with GCC

This was regressed in adf1561d6c.  Since gcc does not support
`__has_feature`, this adjusts the build to use the
`__SANITIZE_ADDRESS__` macro which GCC defines to identify if ASAN is
enabled (similar to `__has_feature`).  This allows building libunwind
with gcc again.

Patch by Daniel Levin!

Reviewed By: compnerd

Differential Revision: https://reviews.llvm.org/D104176

libunwind/src/libunwind.cpp

@@ -16,7 +16,13 @@
 
 #include <stdlib.h>
 
-#if __has_feature(address_sanitizer)
+// Define the __has_feature extension for compilers that do not support it so
+// that we can later check for the presence of ASan in a compiler-neutral way.
+#if !defined(__has_feature)
+#define __has_feature(feature) 0
+#endif
+
+#if __has_feature(address_sanitizer) || defined(__SANITIZE_ADDRESS__)
 #include <sanitizer/asan_interface.h>
 #endif
 
@@ -187,7 +193,7 @@ _LIBUNWIND_WEAK_ALIAS(__unw_get_proc_info, unw_get_proc_info)
 /// Resume execution at cursor position (aka longjump).
 _LIBUNWIND_HIDDEN int __unw_resume(unw_cursor_t *cursor) {
   _LIBUNWIND_TRACE_API("__unw_resume(cursor=%p)", static_cast<void *>(cursor));
-#if __has_feature(address_sanitizer)
+#if __has_feature(address_sanitizer) || defined(__SANITIZE_ADDRESS__)
   // Inform the ASan runtime that now might be a good time to clean stuff up.
   __asan_handle_no_return();
 #endif