cmd/internal/obj/ppc64: support alignment of prefixed insn

Insert machine NOPs when a prefixed instruction crosses a 64B boundary.
ISA 3.1 prohibits prefixed instructions being placed across them. Such
instructions generate SIGILL if executed.

Likewise, adjust the function alignment to guarantee such instructions
can never cross one. And, don't pad the PC based on alignment. The
linker can fit these more optimally.

Likewise, include the function alignment when printing function debug
information. This is needed to verify function alignment happens.

Updates #44549

Change-Id: I434fb0ee4e984ca00dc4566f7569c3bcdf93f910
Reviewed-on: https://go-review.googlesource.com/c/go/+/347050


Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>