doc: update ppc64 section for asm.html

Update the section in asm.html related to PPC64. Remove the line
that says it is in an experimental state, add a link to the
new doc.go file that has all the detail for the Go assembler for
PPC64.

Change-Id: I45d9891669e01d94e2721be576d572e02cd9d2db
Reviewed-on: https://go-review.googlesource.com/c/go/+/183840


Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>