[ARM] Remove FeaturePerfMon from armv7-m

FeaturePerfMon relates to the PMU extensions available in armv7-a, and
should not be available in v7-m (it requires loading from a system
register with a mrc). Sink it down a level in the dependency map so that
it isn't present in ARMv7m or HasV8MMainlineOps.

It is also removed from the Neoverse-N2, as it will already be
transitively included.

Differential Revision: https://reviews.llvm.org/D117022