[AArch64] Introduce the Armv9.5-A architecture version (#72392)

This introduces the Armv9.5-A architecture version, including the
relevant command-line option for -march.

Mode details about the Armv9.5-A architecture version 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/

Patch by Oliver Stannard.