[libFuzzer] Fix Alarm callback in fuchsia.

[charco]

Summary

This patch adds an #if macro to skip the InFuzzingThread() comparison
for fuchsia, similar to what it is done for Windows and NetBSD.

In fuchsia, the alarm callback runs in a separate thread[0], making it fail
the comparison InFuzzingThread(), breaking the -timeout flag.

[0]:
https://github.com/llvm/llvm-project/blob/master/compiler-rt/lib/fuzzer/FuzzerUtilFuchsia.cpp#L323

[jakehehrlich]

LGTM except a nit

compiler-rt/lib/fuzzer/FuzzerLoop.cpp

276–277

Can you mention that Fuchsia does the same thing as windows here?

[charco]

Update comments to keep in sync with code.

[charco]

Thanks for the review!

compiler-rt/lib/fuzzer/FuzzerLoop.cpp

void Fuzzer::InterruptCallback() {
   RmDirRecursive(TempPath(".dir"));
   // Stop right now, don't perform any at-exit actions.
   _Exit(Options.InterruptExitCode);
 }
 
 NO_SANITIZE_MEMORY
 void Fuzzer::AlarmCallback() {
   assert(Options.UnitTimeoutSec > 0);
-  // In Windows Alarm callback is executed by a different thread.
+  // In Windows and Fuchsia, Alarm callback is executed by a different thread.
   // NetBSD's current behavior needs this change too.
-#if !LIBFUZZER_WINDOWS && !LIBFUZZER_NETBSD
+#if !LIBFUZZER_WINDOWS && !LIBFUZZER_NETBSD && !LIBFUZZER_FUCHSIA
   if (!InFuzzingThread())
     return;
 #endif
   if (!RunningUserCallback)
     return; // We have not started running units yet.
   size_t Seconds =
       duration_cast<seconds>(system_clock::now() - UnitStartTime).count();
   if (Seconds == 0)