runtime: wrap darwin libc calls to keep profiler happy

The profiler reports "ExternalCode" when a profiler interrupt happens
while in libc code. Instead, keep track of the most recent Go frame
for the profiler to use.

There is a test for this using time.Now (runtime.TestTimePprof),
which will work once time.Now is moved to using libc (my next CL).

Change-Id: I940ea83edada482a482e2ab103d3a65589979464
Reviewed-on: https://go-review.googlesource.com/114798


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>