cmd/prof, libmach: delete

cmd/prof, libmach: delete

We have never released cmd/prof and don't plan to.
Now that nm, addr2line, and objdump have been rewritten in Go,
cmd/prof is the only thing keeping us from deleting libmach.

Delete cmd/prof, and then since nothing is using libmach, delete libmach.

13,000 lines of C deleted.

LGTM=minux.ma
R=golang-codereviews, minux.ma
CC=golang-codereviews, iant, r
https://golang.org/cl/87020044