[Arm][AArch64] Add support for v8.9-A/v9.4-A base extensions

[Arm][AArch64] Add support for v8.9-A/v9.4-A base extensions

This implements the base extensions that are part of the v8.9-A and
v9.4-A architecture versions, including:

* The Clear BHB Instruction (FEAT_CLRBHB)
* The Speculation Restriction Instruction (FEAT_SPECRES2)
* The SLC target for the PRFM instruction
* New system registers:
  * ID_AA64PFR2_EL1
  * ID_AA64MMFR3_EL1
  * HFGITR2_EL2
  * SCTLR2_EL3

More information on the new extensions can be found on:

* https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/arm-a-profile-architecture-2022
* https://developer.arm.com/downloads/-/exploration-tools

Contributors: Sam Elliott, Tomas Matheson and Son Tuan Vu.

Reviewed By: lenary

Differential Revision: https://reviews.llvm.org/D139424