[Fuzzer] Make InterruptHandler non-blocking for Fuchsia

The initial naive approach to simulate SIGINT on Fuchsia was to getchar
and look for ETX. This caused the InterruptHandler thread to lock stdin,
preventing musl's exit() from being able to close the stdio descriptors
and complete. This change uses select() instead.

Patch By: aarongreen

Differential Revision: https://reviews.llvm.org/D45636

llvm-svn: 330328