[X86] [ABI] Fix i386 ABI "__m64" type bug

According to System V i386 ABI: the  __m64 type paramater and return
value are passed by MMX registers. But current implementation treats
__m64 as i64 which results in parameter passing by stack and returning
by EDX and EAX.

This patch fixes the bug (https://bugs.llvm.org/show_bug.cgi?id=41029)
for Linux and NetBSD.

Patch by Wei Xiao (wxiao3)

Differential Revision: https://reviews.llvm.org/D59744

llvm-svn: 363116