[compiler-rt][hwasan][fuchsia] Implement TagMemoryAligned for fuchsia

[leonardchan]

Summary

This will just be a call into __sanitizer_fill_shadow defined in the fuchsia source tree. This depends on D105663.

[mcgrathr]

lgtm

compiler-rt/lib/hwasan/hwasan_fuchsia.cpp

 }
 
 static void ThreadExitHook(void *hook, thrd_t self) {
   Thread *thread = static_cast<Thread *>(hook);
   atomic_signal_fence(memory_order_seq_cst);
   hwasanThreadList().ReleaseThread(thread);
 }
 
+uptr TagMemoryAligned(uptr p, uptr size, tag_t tag) {
+  CHECK(IsAligned(p, kShadowAlignment));
+  CHECK(IsAligned(size, kShadowAlignment));
+  __sanitizer_fill_shadow(p, size, tag,
+                          common_flags()->clear_shadow_mmap_threshold);
+  return AddTagToPointer(p, tag);
+}
+
 // Not implemented because Fuchsia does not use signal handlers.
 void HwasanOnDeadlySignal(int signo, void *info, void *context) {}
 
 // Not implemented because Fuchsia does not use interceptors.
 void InitializeInterceptors() {}
 
 // Not implemented because this is only relevant for Android.
 void AndroidTestTlsSlot() {}