[mips] Handle transparent unions correctly.

Summary:
This fixes MultiSource/Applications/lemon on big-endian N32 by correcting the
handling of the argument to wait(). glibc defines it as a transparent union of
void* and int*. Such unions are passed according to the rules of the first
member so the argument must be passed as if it were a void* (sign extended from
i32 to i64) and not as a union (shifted to the upper bits of an i64).

wait() already behaves correctly on big-endian O32 and N64 since the union is
already the same size as an argument slot.

Reviewers: atanasyan

Reviewed By: atanasyan

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6963

llvm-svn: 225981