cmd/link, cmd/internal/obj/loong64: support the PCALIGN directive

Allow writing `PCALIGN $imm` where imm is a power-of-2 between 8 and
2048 (inclusive), for ensuring that the following instruction is
placed at an imm-byte boundary relative to the beginning of the
function. If the PC is not sufficiently aligned, NOOPs will be
inserted to make it so, otherwise the directive will do nothing.

This could be useful for both asm performance hand-tuning, and future
scenarios where a certain bigger alignment might be required.

Change-Id: Iad6244669a3d5adea88eceb0dc7be1af4f0d4fc9
Reviewed-on: https://go-review.googlesource.com/c/go/+/479815


Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: WANG Xuerui <git@xen0n.name>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>