[compiler-rt] [compiler-rt][AArch64] Don't use x18 in __arm_sme_save (PR #157802)

[Peter Smith via llvm-commits]

https://github.com/smithp35 approved this pull request.

Thanks for the patch. LGTM, especially as x16 is a simple substitution.

AAPCS64 closest link to platform register usage https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#611general-purpose-registers



https://github.com/llvm/llvm-project/pull/157802