[AArch64] Add SME 2 target feature for Armv8-A and Armv9-A 2022 Architecture Extension

[AArch64] Add SME 2 target feature for Armv8-A and Armv9-A 2022 Architecture Extension

First patch in a series adding MC layer support for Scalable Matrix
Extension 2 (SME2).

This patch adds the following feature:
  sme2

The 2022 Architecture Extension release adds other feature flags(eg.:sme2.1),
that will be in follow-up patches.

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

Differential Revision: https://reviews.llvm.org/D135448