[dev.power64] cmd/9a, cmd/9c, cmd/9l, liblink: import code from Vita Nuova.

No modifications other than adding copyright header to each file, and
concatenating several cmd/9l files together to form the liblink files.

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://codereview.appspot.com/123840043
[dev.power64] cmd/9g: add doc.go to prevent build failures.

After CL 117670045, cmd/dist will expect to have a cmd/9g directory.

LGTM=rsc, dave
R=rsc, iant, dave
CC=golang-codereviews
https://codereview.appspot.com/126780043