cmd/internal/obj/loong64: change the plan9 format of the prefetch instruction PRELDX

cmd/internal/obj/loong64: change the plan9 format of the prefetch instruction PRELDX

before:
    MOVV    $n + $offset, Roff
    PRELDX  (Rbase)(Roff), $hint
after:
    PRELDX  offset(Rbase), $n, $hint

This instruction is supported in CL 671875, but is not actually used

Change-Id: I943d488ea6dc77781cd796ef480a89fede666bab
Reviewed-on: https://go-review.googlesource.com/c/go/+/673155
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>