[libFuzzer] Fix Alarm callback in fuchsia.

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

Author: charco (aka Marco  Vanotti)
Differential Revision: https://reviews.llvm.org/D68166

llvm-svn: 374228