[TestMiVar] Enable one of the tests for GCC.
The "internal" name of vars is different between clang and GCC. All this
change does is to use a regex instead of the hardcoded internal name.
Test Plan: dotest.py -C <clang|gcc> -p TestMiVar
Reviewed By: ki.stfu
Differential Revision: http://reviews.llvm.org/D9128