cmd/compile: add ABI register definations for loong64

cmd/compile: add ABI register definations for loong64

Updates #40724

Co-authored-by: Xiaolin Zhao <zhaoxiaolin@loongson.cn>
Change-Id: I56f7382dda58a565b8c3256f1c7845a3031f67de
Reviewed-on: https://go-review.googlesource.com/c/go/+/521776
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: David Chase <drchase@google.com>
Auto-Submit: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>