chThread: support Mac OS X

Mac OS X strangely has the POSIX semaphore API but doesn't implement it
correctly. I really love seeing funky and hard-to-debug return values instead
of compile errors. Thanks, Apple!

We basically need to use named semaphores instead of the unnamed semaphores
that every other POSIX-compliant platform supports.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
chTimer: add Mac OS X support via mach_absolute_time

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody: make OpenMP version optional

Since clang++ doesn't have OpenMP support, it makes some sense to have an
ability to exclude OpenMP from the build.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>