[libfuzzer][MSVC] Make calls to builtin functions work with MSVC
Replace calls to builtin functions with macros or functions that call the
Windows-equivalents when targeting windows and call the original
builtin functions everywhere else.
This change makes more parts of libFuzzer buildable with MSVC.
Reviewed By: vitalybuka
Subscribers: mgorny, rnk, thakis
Differential Revision: https://reviews.llvm.org/D56439