When the test is executed in user mode qemu for mips64, it fails because
the buffer is too small. Increasing the buffer fixes the problem.

BUG=
TEST=cctest/test-mark-compact/RegressJoinThreadsOnIsolateDeinit

Review-Url: https://codereview.chromium.org/2416573002
Cr-Commit-Position: refs/heads/master@{#40278}