[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, making it fail
the comparison InFuzzingThread(), breaking the -timeout flag.
Author: charco (aka Marco Vanotti)
Differential Revision: https://reviews.llvm.org/D68166