runtime/pprof: test multithreaded profile, remove OS X workarounds

This means that pprof will no longer report profiles on OS X.
That's unfortunate, but the profiles were often wrong and, worse,
it was difficult to tell whether the profile was wrong or not.

The workarounds were making the scheduler more complex,
possibly caused a deadlock (see issue 5519), and did not actually
deliver reliable results.

It may be possible for adventurous users to apply a patch to
their kernels to get working results, or perhaps having no results
will encourage someone to do the work of creating a profiling
thread like on Windows. Issue 6047 has details.

Fixes issue 5519.
Fixes issue 6047.

R=golang-dev, bradfitz, r
CC=golang-dev
https://codereview.appspot.com/12429045