Added return value registers on both MIPS32 and MIPS 64.

Bug: 
Change-Id: I3199a73f90ef7b0bc9e18d716fcdf4c8c479f7fd
Reviewed-on: https://chromium-review.googlesource.com/774286Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#49438}