[dev.power64] 5g,6g,8g: synchronize documentation for regopt structures

I added several comments to the regopt-related structures when
porting it to 9g.  Synchronize those comments back in to the
other compilers.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/175720043
[dev.power64] 5g,6g,8g,9g: debug prints for regopt pass 6 and paint2

Theses were very helpful in understanding the regions and
register selection when porting regopt to 9g.  Add them to the
other compilers (and improve 9g's successor debug print).

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/174130043
[dev.power64] liblink: generate dnames[5689] for D_* constants

This is more complicated than the other enums because the D_*
enums are full of explicit initializers and repeated values.
This tries its best.  (This will get much cleaner once we
tease these constants apart better.)

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/166700043
[dev.power64] 6g,9g: formatters for Prog and Addr details

The pretty printers for these make it hard to understand
what's actually in the fields of these structures.  These
"ugly printers" show exactly what's in each field, which can
be useful for understanding and debugging code.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/175780043
[dev.power64] liblink: fix Solaris build

a->class is a char.  Boo hoo.

LGTM=minux
R=rsc, minux
CC=golang-codereviews
https://codereview.appspot.com/169630043
[dev.cc] all: merge dev.power64 (f57928630b36) into dev.cc

This will be the last dev.power64 merge; we'll finish on dev.cc.

TBR=austin
CC=golang-codereviews
https://codereview.appspot.com/175420043