cmd/dist: only build ios_exec wrapper for ios
cmd/dist: only build ios_exec wrapper for ios

Not for darwin/arm64.

Updates #38485.

Change-Id: I08a5f00fd77f20c9c483755a36755a63cf10aa1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/262558
Trust: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>

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