[AArch64][SME] Add system registers and related instructions

This patch adds the new system registers introduced in SME:

  - ID_AA64SMFR0_EL1 (ro) SME feature identifier.
  - SMCR_ELx (r/w) streaming mode control register for configuring
    effective SVE Streaming SVE Vector length when the PE is in
    Streaming SVE mode.
  - SVCR (r/w) streaming vector control register, visible at all
    exception levels. Provides access to PSTATE.SM and PSTATE.ZA
    using MSR and MRS instructions.
  - SMPRI_EL1 (r/w) streaming mode execution priority register.
  - SMPRIMAP_EL2 (r/w) streaming mode priority mapping register.
  - SMIDR_EL1 (ro) streaming mode identification register.
  - TPIDR2_EL0 (r/w) for use by SME software to manage per-thread
    SME context.
  - MPAMSM_EL1 (r/w) MPAM (v8.4) streaming mode register, for
    labelling memory accesses performed in streaming mode.

Also added in this patch are the SME mode change instructions.
Three MSR immediate instructions are implemented to set or clear
PSTATE.SM, PSTATE.ZA, or both respectively:

  - MSR SVCRSM, #<imm1>
  - MSR SVCRZA, #<imm1>
  - MSR SVCRSMZA, #<imm1>

The following smstart/smstop aliases are also implemented for
convenience:

  smstart    -> MSR SVCRSMZA, #1
  smstart sm -> MSR SVCRSM,   #1
  smstart za -> MSR SVCRZA,   #1

  smstop     -> MSR SVCRSMZA, #0
  smstop sm  -> MSR SVCRSM,   #0
  smstop za  -> MSR SVCRZA,   #0

The reference can be found here:
https://developer.arm.com/documentation/ddi0602/2021-06

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D105576

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