cmd/asm, runtime: remove the RSB register from loong64

cmd/asm, runtime: remove the RSB register from loong64

It was carryover from the mips64 port (where it represented the platform
GP register) but LoongArch platform ABI doesn't have the GP concept.

Change-Id: Iea326ae13676e95b040b52aaadc08d311b507bd3
Reviewed-on: https://go-review.googlesource.com/c/go/+/475815
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>