cmd/compile, cmd/internal, runtime: change the registers used by the duff device...

cmd/compile, cmd/internal, runtime: change the registers used by the duff device for loong64

Add R21 to the allocatable registers, use R20 and R21 in duff
device. This CL is in preparation for subsequent regABI support.

Updates #40724

Co-authored-by: Xiaolin Zhao <zhaoxiaolin@loongson.cn>
Change-Id: If1661adc0f766925fbe74827a369797f95fa28a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/521775
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>