cmd/internal/obj/loong64: add CPUCFG instructions support

cmd/internal/obj/loong64: add CPUCFG instructions support

The CPUCFG instruction is used to dynamically obtain the features
supported by the current CPU during the running of the program.

Go asm syntax:
	CPUCFG RJ, RD

Equivalent platform assembler syntax:
	cpucfg rd, rj

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

Change-Id: I545110ff837ae9c5ccd7c448a1daf2d1277f9aa1
Reviewed-on: https://go-review.googlesource.com/c/go/+/493436
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Qiqi Huang <huangqiqi@loongson.cn>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>