MIPS: Fix memory barriers for atomic operations.

Add barriers using MIPS 'sync' instructions as needed for SMP
systems.

BUG=246947
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/15981017

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15062 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
MIPS: Allow timeout for TerminateMultipleV8ThreadsDefaultIsolate.

test-thread-termination/TerminateMultipleV8ThreadsDefaultIsolate
times out on the MIPS simulator.

Allow the timeouts until this is fixed.

BUG=v8:2657
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/16203005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15063 ce2b1a6d-e550-0410-aec6-3dcde31c8c00