[sanitizer_common] Rename SI_NOT_WINDOWS to SI_POSIX

Summary:
New systems might be neither Windows nor POSIX.  The SI_NOT_WINDOWS
macro in sanitizer_platform_interceptors.h was already effectively
the same as SI_POSIX, so just use SI_POSIX instead.

Submitted on behalf of Roland McGrath.

Reviewers: vitalybuka, alekseyshl, kcc

Reviewed By: vitalybuka

Subscribers: phosek, filcab, llvm-commits, kubamracek

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D36038

llvm-svn: 309536