[clang] Limit clang test to ARM and AArch64 only

Summary: Limit clang/test/Frontend/gnu-mcount.c to ARM and AArch64 only.

Reviewers: abdulras, honggyu.kim, rengolin

Subscribers: aemerson, rengolin, cfe-commits

Tags: #clang-c

Differential Revision: https://reviews.llvm.org/D25842

llvm-svn: 284931