cmd/internal/obj/arm64: optimize constant pool for 32-bit constants

cmd/internal/obj/arm64: optimize constant pool for 32-bit constants

Current assembler encodes "ADD $0xaaaaaaaa, Rx" to "MOVD off(PC), Rtmp"
 + "ADD Rtmp, Rx", and a 64-bit item is stored in the constant pool.

This patch optimizes it to "MOVWU off(PC), Rtmp" + "ADD Rtmp, Rx",
and a 32-bit item is stored.

The total size of the executable binary go and the library files in
pkg/linux_arm64 decreased about 3KB by this patch.

Change-Id: Ieb1592f78ef9ed52f5d3ad232d6cdf87d0923de1
Reviewed-on: https://go-review.googlesource.com/107516
Reviewed-by: Wei Xiao <Wei.Xiao@arm.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>