[mips] Remove Triple:mips from SupportedArchs in MCJIT unittests

MIPS does not handle multiple relocations correctly, so two tests from the
unittests are expected to fail. These are:
 - MCJITTest.return_global and
 - MCJITTest.multiple_functions.

Until the multiple relocations are fixed, XFAIL the MCJIT unittests for
MIPS. This issue is tracked as Bug 16250.

Patch by Petar Jovanovic.