cmd/asm: process forward jump to PCALIGN

cmd/asm: process forward jump to PCALIGN

The forward jump target are not processed when the target is PCALIGN, so
process it before emit nops for PCALIGN.

Fixes #74648

Change-Id: I690fbfacf79e26d7a37628a2551729b2381616c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/696915
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>