MIPS64: Use weak cell in LoadGlobal handler.

Port bc0d2e237b1b624bed26ba849529b40329ea72a2

BUG=v8:3629
LOG=N
TBR=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/814193004

Cr-Commit-Position: refs/heads/master@{#25906}