cmd/compile: sort regalloc switch by architecture
cmd/compile: sort regalloc switch by architecture

Also tweak comment for the arm64 case.

Change-Id: I073405bd2acf901dcaaf33a034a84b6a09dd4a83
Reviewed-on: https://go-review.googlesource.com/c/go/+/334869
Trust: Joel Sing <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@sing.id.au>
Reviewed-by: Meng Zhuo <3e6d300a356a3b86bc195677414a9ec5508ca893@golangcn.org>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

cloudflare/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,golang/go,golang/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,golang/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go