When using c-archive/c-shared, the signal handler for SIGPROF will not
be installed, which means that runtime/pprof.StartCPUProfile won't work.
There is no really good solution here, as the main program may want to
do its own profiling.  For now, just document that runtime/pprof doesn't
work as expected, but that it will work if you use Notify to install the
Go signal handler.

Fixes #14043.

Change-Id: I7ff7a01df6ef7f63a7f050aac3674d640a246fb4
Reviewed-on: https://go-review.googlesource.com/18911
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>