[clang] [llvm] [arm] Support reserving r4 and r5 alongside r9 (PR #89849)

[via cfe-commits]

benisxdxd wrote:

@smithp35 The system i'm interacting with has a GCC plugin that uses r4 and r5 for custom software mitigations against exploitation. 
I need a way to inform LLVM to keep this registers untouched so I don't interfere and cause a corruption.
By reserving those registers I will be able to compile and integrate rust code to that platform, a first step in a long way.
>From what I saw, r4 or r5 are general purpose in LLVM because if it was not then I assume we would see something in `getReservedRegs` of ARM.
If I have missed something I would happy for a referral to the right location.

https://github.com/llvm/llvm-project/pull/89849