[AArch64] Add SME2.1 target feature for Armv9-A 2022 Architecture Extension

First patch in a series adding MC layer support for SME2.1.

This patch adds the following feature:

sme2p1

The reference can be found here:
https://developer.arm.com/documentation/ddi0602/2022-09

Differential Revision: https://reviews.llvm.org/D137410