[AArch64] Assembly support for Armv9.5-A Debug/PMU Extensions (#73537)

This implements assembly support for the Debug/PMU extensions introduced
as part of the Armv9.5-A architecture version.
The changes include:
* New ID_AA64DFR2_EL1 ID system register
* New MDSTEPOP_EL1 and SPMZR_EL0 system registers

Mode details about these extensions can be found at:
* https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/arm-a-profile-architecture-developments-2023
* https://developer.arm.com/documentation/ddi0602/2023-09/



Co-authored-by: Oliver Stannard <oliver.stannard@arm.com>