[MIPS] Correctly set $pc and $ra in NaClThreadContextToSignalContext
[MIPS] Correctly set $pc and $ra in NaClThreadContextToSignalContext

Program counter and return address have not been correctly set in
NaClThreadContextToSignalContext. This caused two tests to fail:

- run_thread_suspension_test and
- run_gdb_syscall_thread_test.

This change corrects these failures.

NOTRY=true
However, we have run a try job manually, and it passed fine, except for
the nacl-lucid64_validator_opt that is broken.

BUG= the aforementioned tests
TEST= run the failing tests

Review URL: https://chromiumcodereview.appspot.com/19572004

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@11784 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client