[MINGW] normalize WIN32 macros

move _WIN64 and _WIN32 defines to lib/Basic/Targets/OSTargets.h
move WIN32, WIN64 and __MINGW64__ to addMinGWDefines

fixes __MINGW64__ not being defined for aarch64
adds WIN32 definition for x64

Reviewers: mstorsjo

Differential Revision: https://reviews.llvm.org/D40285

llvm-svn: 318755