sanitizer: support GCC's fallthrough attribute

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

when being built with GCC.

Differential Revision: https://reviews.llvm.org/D124832

Author: marxin

compiler-rt/lib/sanitizer_common/sanitizer_internal_defs.h

@@ -259,7 +259,9 @@ typedef u64 tid_t;
 # 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