[Clang][AArch64] Implement ACLE feature macro for FEAT_LRCPC3

This implements the new value for the `__ARM_FEATURE_RCPC` feature
macro, which was introduced to the ACLE to indicate the availability of
FEAT_LRCPC3.

More details can be found on:
https://github.com/ARM-software/acle/blob/main/main/acle.md#rcpc

Reviewed By: tmatheson

Differential Revision: https://reviews.llvm.org/D153130