[llvm] [AArch64] Avoid raw_svector_ostream for SubtargetMap keys (PR #196003)

[Alexis Engelke via llvm-commits]

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

LGTM. raw_svector_ostream is known to be inefficient.

That said, most functions have the same target attributes and therefore this always return the same value.. would it be worth optimizing for that case?

https://github.com/llvm/llvm-project/pull/196003