Sigh. Sleeps on FreeBSD also yield the rest of the time slice and
profiling signals are only delivered when a process completes a time
slice (worse, itimer time is only accounted to the process that
completes a time slice). It's less noticeable than the other BSDs
because the default tick rate is 1000Hz, but it's still failing
regularly.

Fixes #13846.

Change-Id: I41bf116bffe46682433b677183f86944d0944ed4
Reviewed-on: https://go-review.googlesource.com/18455Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Austin Clements <austin@google.com>