[Driver] Split up huge arm-cortex-cpus.c test.

This test file has grown to the point where it takes a huge amount of
time to run. At the moment, this test seems to consistently time out
when running in the pre-commit checks in Phabricator with a 10 minute
timeout. For example see
https://reviews.llvm.org/harbormaster/unit/view/2832724/

While splitting up the test file is not ideal, it is even more
undesirable to have huge test files that time out in common settings.

This patch splits up the test file roughly in the middle.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D120876