[libFuzzer] AlrmHandler is executed in a different thread for Windows.

Don't check for InFuzzingThread() on Windows, since the AlarmHandler() is
always executed by a different thread from a thread pool.
If we don't add these changes, the alarm handler will never execute.
Note that we decided to ignore possible problem in the synchronization.

Differential Revision: https://reviews.llvm.org/D28723

llvm-svn: 292746