[libFuzzer] Ignore synthetic exceptions on Fuchsia

Fuchsia has several exceptions which are merely informational and should
not be treated as crashes. This patch puts logic that read from the
exception port and handled the exception in a loop, and ignores
non-exceptions and informational exceptions.

Patch By: aarongreen

Differential Revision: https://reviews.llvm.org/D62226

llvm-svn: 361407