Compile and test i128 math builtins for Win64

Summary:
Windows has always been LLP64, not LP64, so the macros were incorrect.
Check for _WIN64, since AArch64 sets that too.

The tests have to be fixed up in two main ways:
1. Use 'ULL' suffixes to avoid sign extension when passing hex literals
   with the sign bit set to signed 128 arguments. Clang
   -fms-compatibility makes such hex literals signed, not unsigned.
2. Disable various tests for 80-bit long double interoperation with i128
   values.

Reviewers: joerg, mstorsjo

Subscribers: javed.absar, kristof.beyls, hiraditya, aheejin, llvm-commits

Differential Revision: https://reviews.llvm.org/D53918

llvm-svn: 345796