cmd/asm: add support for bdnz/bdz extended mnemonics on PPC64

Support BDNZ and BDZ mnemonics, they are commonly used
POWER instructions. The raw BC mnemonic is not easy
to read.

Likewise, cleanup code surrounding these changes.

Change-Id: I72f1dad5013f7856bd0dd320bfb17b5a9f3c69ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/390696


Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Trust: Paul Murphy <murp@ibm.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>