[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