cmd/compile: enable reg args on riscv64
cmd/compile: enable reg args on riscv64

This CL updates config.go to enable register args.

Change-Id: I00697fc3db23293be0f5bd2fe33fb0055eeab43e
Reviewed-on: https://go-review.googlesource.com/c/go/+/360217
Trust: mzh <3e6d300a356a3b86bc195677414a9ec5508ca893@golangcn.org>
Run-TryBot: mzh <3e6d300a356a3b86bc195677414a9ec5508ca893@golangcn.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

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