[MS] Make __iso_volatile_* available on all targets

Future versions of MSVC make these intrinsics available on x86 & x64,
according to:
http://lists.llvm.org/pipermail/cfe-dev/2019-March/061711.html

The purpose of these builtins is to emit plain, non-atomic, volatile
stores when /volatile:ms (-cc1 -fms-volatile) is enabled.

llvm-svn: 357220