Fix breakage on V8 Linux - nosse2

This was introduced by https://code.google.com/p/v8/source/detail?r=20857

R=adamk@chromium.org
TBR=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/241433002

Patch from Erik Arvidsson <arv@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20858 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
MIPS: Serializer enable/disable flags need thread safety.

Port r20855 (ac70e129)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/239173004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20859 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Merge remote-tracking branch 'upstream/bleeding_edge' into bleeding_edge