Fix test/CodeGen/mips-varargs.c to use %clang_cc1

Only tests under test/Driver should use %clang, and test/CodeGen in
particular must always use %clang_cc1.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@218260 91177308-0d34-0410-b5e6-96231b3b80d8