[AArch64] Add missing v8.8a Non-maskable Interrupts feature

This adds support for the missing Non-maskable Interrupts (FEAT_NMI)
feature from armv8.8-A, which consists of the `ALLINT` pstate register.

This is a second iteration of the patch from D131389, building on top of
the D139925 changes that enable better support for `msr (immediate)`
instructions that take 1-bit immediates.

Contributors:
* David Candler
* Tomas Matheson
* Sam Elliott

Reviewed By: lenary, tmatheson

Differential Revision: https://reviews.llvm.org/D140216