cmd/internal/obj/loong64: add atomic memory access instructions support

cmd/internal/obj/loong64: add atomic memory access instructions support

The AM* atomic access instruction performs a sequence of “read-modify-write”
operations on a memory cell atomically. Specifically, it retrieves the old
value at the specified address in memory and writes it to the general register
rd, performs some simple operations on the old value in memory and the value
in the general register rk, and then write the result of the operation back
to the memory address pointed to by general register rj.

Go asm syntax:
	AM{SWAP/ADD/AND/OR/XOR/MAX/MIN}[DB]{W/V} RK, (RJ), RD
	AM{MAX/MIN}[DB]{WU/VU} RK, (RJ), RD

Equivalent platform assembler syntax:
	am{swap/add/and/or/xor/max/min}[_db].{w/d} rd, rk, rj
	am{max/min}[_db].{wu/du} rd, rk, rj

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

Change-Id: I99ea4553ae731675180d63691c19ef334e7e7817
Reviewed-on: https://go-review.googlesource.com/c/go/+/481577
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Qiqi Huang <huangqiqi@loongson.cn>
Reviewed-by: WANG Xuerui <git@xen0n.name>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>