[LibFuzzer] Add macro flags for Posix and Windows.

This is the beginning of an effort to get libfuzzer working on
Windows.  This is a NFC to just add some macros for platform
detection on Windows.

Patch by Marcos Pividori
Differential Revision: https://reviews.llvm.org/D27229

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@288249 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed