Adding some details on writing Go assembler for ppc64.

Change-Id: I46fc6b75ee6c36946f90280b2b670e0d32bcc6b1
Reviewed-on: https://go-review.googlesource.com/c/go/+/183837Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>