cmd/internal/obj/arm: fix setting U bit in shifted register offset of MOVBS

cmd/internal/obj/arm: fix setting U bit in shifted register offset of MOVBS

"MOVBS.U R0<<0(R1), R2" is assembled to 0xe19120d0 (ldrsb r2, [r1, r0]),
but it is expected to be 0xe11120d0 (ldrsb r2, [r1, -r0]).

This patch fixes it and adds more encoding tests.

fixes #20701

Change-Id: Ic1fb46438d71a978dbef06d97494a70c95fcbf3a
Reviewed-on: https://go-review.googlesource.com/45996
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>