cmd/link: add relocation type R_AARCH64_LDST16_ABS_LO12_NC for arm64

The linker already has R_AARCH64_LDST{8,32,64,128}_ABS_LO12_NC, some cgo tests require
 R_AARCH64_LDST16_ABS_LO12_NC, this CL adds this relocation type.

Fixes #42660

Change-Id: I9a5120cd872f5095c61175cb602427c6ab3225cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/271017


Reviewed-by: eric fang <eric.fang@arm.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: eric fang <eric.fang@arm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: eric fang <eric.fang@arm.com>
Trust: Benny Siegert <bsiegert@gmail.com>