prof: install gopprof too

prof: install gopprof too

This is a modified version of the open source pprof
from code.google.com/p/google-perftools.
That version is likely to catch up to this one,
but it's still useful to ship our own copy since
we only need the one script from that project,
not all the C++ libraries.

R=r
CC=golang-dev
https://golang.org/cl/783041