[MIPS] Add support for thread_suspension test

This change:

- implements NaClSignalContextUnsetClobberedRegisters() for MIPS,
- modifies Unwind for thread suspension to match what is needed for MIPS,
- adds MIPS-arch specific parts in suspend_test_guest.c and
  suspend_test_host.c.

BUG= run_thread_suspension_test fails for MIPS
TEST= run run_thread_suspension_test on MIPS-board

Review URL: https://codereview.chromium.org/121833002

git-svn-id: svn://svn.chromium.org/native_client/trunk/src/native_client@12571 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2
5 files changed