[llvm] ARM: Remove check for isAAPCS_ABI when enabling various aeabi calls (PR #152108)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

We can "remove" support for -mabi without actually removing the flag; just only allow -mabi specifications that don't actually change the ABI.

https://github.com/llvm/llvm-project/pull/152108