cmd/internal/obj/loong64: add support for instructions BSTRPICK.{W/D} and BSTRINS...

cmd/internal/obj/loong64: add support for instructions BSTRPICK.{W/D} and BSTRINS.{W/D}

Go asm syntax:
	BSTRPICK{W/V}	$msb, RJ, $lsb, RD
	BSTRINS{W/V}	$msb, RJ, $lsb, RD

Equivalent platform assembler syntax:
	bstrpick.{w/d}	rd, rj, $msb, $lsb
	bstrins.{w/d}	rd, rj, $msb, $lsb

Ref: https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html

Change-Id: I8b89b766ed22a96da7d8d5b2b2873382a49208de
Reviewed-on: https://go-review.googlesource.com/c/go/+/604735
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>