[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