compiler-rt/test/metadata: restrict tests to x86_64

aarch64 builder broke with:

-- Supported architectures for crt: aarch64
CMake Error at compiler-rt/cmake/config-ix.cmake:244 (message):
  Unsupported architecture: x86_64
Call Stack (most recent call first):
  compiler-rt/cmake/config-ix.cmake:280 (get_target_flags_for_arch)
  compiler-rt/test/metadata/CMakeLists.txt:7 (get_test_cc_for_arch)
-- Configuring incomplete, errors occurred!

https://lab.llvm.org/buildbot/#/builders/223/builds/11423

Restrict the tests to x86_64 only.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D139323