MIPS: Silence warnings and unbreak optdebug build for target.

This is a temporary workaround for bogus array-bounds warning, due to bug in MIPS toolchain versions 4.8.2 and 4.9.2.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56273

TEST=
BUG=

Review URL: https://codereview.chromium.org/928533002

Cr-Commit-Position: refs/heads/master@{#26756}