sanitizer: support GCC's fallthrough attribute

[marxin]

Summary

Fixes:
sanitizer_stack_store.cpp:257:13: warning: this statement may fall through [-Wimplicit-fallthrough=]

when being built with GCC.

[marxin]

It caused:

/b/sanitizer-x86_64-linux/build/llvm-project/compiler-rt/lib/sanitizer_common/sanitizer_stack_store.cpp:258:7: error: use of the 'fallthrough' attribute is a C++17 extension [-Werror,-Wc++17-attribute-extensions]
      FALLTHROUGH;
      ^
b/sanitizer-x86_64-linux/build/llvm-project/compiler-rt/lib/sanitizer_common../sanitizer_common/sanitizer_internal_defs.h:263:25: note: expanded from macro 'FALLTHROUGH'
#  define FALLTHROUGH [[fallthrough]]

Which is similar to:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105377

The warning is misleading as it's guarded in __has_cpp_attribute.

[dvyukov]

If this is a new warning, maybe reordering these will help:

#if has_cpp_attribute(fallthrough)
#elif has_cpp_attribute(clang::fallthrough)

[marxin]

Yep, that works. Let me push such change.

[marxin]

Fixed in: https://github.com/llvm/llvm-project/commit/903b8845fb5a0a366266d6f28231f994a5721178

compiler-rt/lib/sanitizer_common/sanitizer_internal_defs.h

 #endif
 
 #if !defined(_MSC_VER) || defined(__clang__) || MSC_PREREQ(1900)
 # define NOEXCEPT noexcept
 #else
 # define NOEXCEPT throw()
 #endif
 
-#if __has_cpp_attribute(clang::fallthrough)
+#if __has_cpp_attribute(fallthrough)
+#  define FALLTHROUGH [[fallthrough]]
+#elif __has_cpp_attribute(clang::fallthrough)
 #  define FALLTHROUGH [[clang::fallthrough]]
 #else
 #  define FALLTHROUGH
 #endif
 
 // Unaligned versions of basic types.
 typedef ALIGNED(1) u16 uu16;
 typedef ALIGNED(1) u32 uu32;