runtime/pprof: output CPU profiles in pprof protobuf format

runtime/pprof: output CPU profiles in pprof protobuf format

This change buffers the entire profile and converts in one shot
in the profile writer, and could use more memory than necessary
to output protocol buffer formatted profiles. It should be
possible to convert each chunk in a stream (maybe maintaining
some minimal state to output in the end) which could save on
memory usage.

Fixes #16093

Change-Id: I946c6a2b044ae644c72c8bb2d3bd82c415b1a847
Reviewed-on: https://go-review.googlesource.com/33071
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>