[X86] Properly implement the calling convention for f80 for mingw/x86_64

In these cases, both parameters and return values are passed
as a pointer to a stack allocation.

MSVC doesn't use the f80 data type at all, while it is used
for long doubles on mingw.

Normally, this part of the calling convention is handled
within clang, but for intrinsics that are lowered to libcalls,
it may need to be handled within llvm as well.

Differential Revision: https://reviews.llvm.org/D44592

llvm-svn: 327957