Most platforms do not need these methods. Thus, make them private to
the mips headers.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I3fb1a2a3fd9a53dfc55b45763c150911db43b537
Reviewed-on: https://chromium-review.googlesource.com/1169203Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55032}