[X86] Move atomics test for !pcsections into separate file

The pcsections.ll test primarily tests that the AsmPrinter produces the
right output in sections. This output is not easily covered by
update_llc_test_checks.py, and as such is hand written. This makes
maintenance rather burdensome. Instead, let's keep pcsections.ll as
simple as possible.

Move the more complex tests that primarily test that some atomic
operations end up in the PC section to pcsections-atomics.ll.

NFC.

Reviewed By: dvyukov, vitalybuka

Differential Revision: https://reviews.llvm.org/D144675