Subzero: Add the MIPS=1 makefile option for alternate testing mode.

Subzero: Add the MIPS=1 makefile option for alternate testing mode.

There are two problems to address:

1. Google developers lack some tooling to test MIPS changes.

2. MIPS developers lack some tooling/packages to allow full "make presubmit" testing.

Until all this gets sorted out, we add the "make -f Makefile.standalone MIPS=1" option to control some of the testing targets:

check-lit: No changes, as all these lit tests should be runnable in both environments.

check-xtest: MIPS=1 runs just the mips32 tests, plus the x8664 native tests as a sanity-check.  Non-MIPS runs everything except the mips32 tests.

check-spec: MIPS=1 completely disables spec2k testing.

presubmit: MIPS=1 greatly reduces the number of tests (especially since so many are spec variants).

With this change, mips32 CLs should pass "make -f Makefile.standalone MIPS=1 presubmit" before upload, and other CLs should pass "make -f Makefile.standalone presubmit".

BUG= none
R=kschimpf@google.com, obucinac@gmail.com

Review URL: https://codereview.chromium.org/2271053006 .