[compiler-rt] Allow sanitizers to be compiled for windows with clang

Summary:
A few small changes required to permit building the sanitizers
with Clang instead of only with MSVC.

Reviewers: compnerd, beanz, rnk

Subscribers: beanz, timurrrr, kubabrecka, dberris, llvm-commits

Differential Revision: https://reviews.llvm.org/D24092

llvm-svn: 280863