MIPS: Follow-up to "Add two special runtime methods"

This is a MIPS-specific follow-up to
https://android-review.googlesource.com/449659.

Test: booted MIPS64 (with 2nd arch MIPS32R6) in QEMU
Test: test-art-target-gtest
Test: testrunner.py --target --optimizing
Test: same tests as above on CI20
Test: booted MIPS32R2 in QEMU

Bug: 33616143

Change-Id: I6428e7dccf2a5dcd0dd1135cf11cbcc0cf201c27