This adds subtarget features for v8.7-A's limited-tlb-maintenance nXS
instruction qualifiers, for the WFET/WFIT instructions and for the
HCRX_EL2 system register, allowing each to be conditional on their own
SubtargetFeature instead of being conditional on the base "v8.7a" feature.
Based on patches written by Simon Tatham.
Why is this FeatureHCX, and not FeatureHCRX for consistency with the register name?