Fixed a few errors in implementation of FastNewObjectStub::Generate
for MIPS64 that cause a hadnfull of tests to fail.

TEST=cctest/test-heap-profiler/TrackBumpPointerAllocations
BUG=

Review URL: https://codereview.chromium.org/1773533002

Cr-Commit-Position: refs/heads/master@{#34535}