Originally, we default to use load and store instruction with 20 bit displacement.
However, that is not necessary. Some instructions have a displacement smaller
than 12 bit. This CL allows the usage of 12 bit load and store instruction when
that happens.

This change also reduces the size of .text section in go binary by 19 KB.

Some tests are also added to verify the functionality of the change.

Change-Id: I13edea06ca653d4b9ffeaefe8d010bc2f065c2ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/194857Reviewed-by: Michael Munday <mike.munday@ibm.com>
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>