Separate builtins for x84-64 and i386; implement __mulh and __umulh

Summary: We need x86-64-specific builtins if we want to implement some of the MS intrinsics - winnt.h contains definitions of some functions for i386, but not for x86-64 (for example _InterlockedOr64), which means that we cannot treat them as builtins for both i386 and x86-64, because then we have definitions of builtin functions in winnt.h on i386.

Reviewers: thakis, majnemer, hans, rnk

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D24598

llvm-svn: 283264