cmd/internal/obj/loong64: add FLDX,FSTX,LDX.STX instructions support

cmd/internal/obj/loong64: add FLDX,FSTX,LDX.STX instructions support

The LDX.{B,BU,H,HU,W,WU,D},STX.{B,H,W,D}, FLDX.{S,D}, FSTX.{S,D} instruction
on Loong64 implements memory access operations using register offset

Go asm syntax:
	MOV{B,BU,H,HU,W,WU,V}	(RJ)(RK), RD
	MOV{B,H,W,V}		RD, (RJ)(RK)
	MOV{F,D}		(RJ)(RK), FD
	MOV{F,D}		FD, (RJ)(RK)

Equivalent platform assembler syntax:
        ldx.{b,bu,h,hu,w,wu,d}	rd, rj, rk
        stx.{b,h,w,d}		rd, rj, rk
        fldx.{s,d}		fd, rj, rk
        fstx.{s,d}		fd, rj, rk

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

Change-Id: Ic7d13bf45dab8342f034b6469465e6337a087144
Reviewed-on: https://go-review.googlesource.com/c/go/+/588215
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Qiqi Huang <huangqiqi@loongson.cn>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Auto-Submit: abner chenc <chenguoqi@loongson.cn>