[MIPS] Add support for debug stub test

Add MIPS specific parts for this debug stub test, including:

- definition of MIPS_REG_DEFS,
- c code that sets expected register values in untrusted code,
- python code that checks for expected values in registers,
- other definitions and conditions needed for the test.

BUG= run_debug_stub_test for MIPS
TEST= run run_debug_stub_test on MIPS-HW

Review URL: https://codereview.chromium.org/107313005

git-svn-id: svn://svn.chromium.org/native_client/trunk/src/native_client@12570 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2