[AArch64] Fix list of features for Cortex-X1C

This patch fixes the list of subtarget features enabled for the
Cortex-X1C processor, including the following:

* Fix incorrect version used for FeatureRCPC:
  * Use FEAT_LRCPC2 instead of FEAT_LRCPC.
* Add missing v8.4-A features included in the TRM:
  * Flag Manipulation Instructions - FeatureFlagM (FEAT_FlagM)
  * Large System Extension 2 - FeatureLSE2 (FEAT_LSE2)

Reviewed By: vhscampos

Differential Revision: https://reviews.llvm.org/D132120