Implement clock_gettime() for Darwin

which doesn't have it, but it does have a high-resolution monotonic
clock that you can sample and convert into nanoseconds.
A test was including <malloc.h> which Darwin doesn't have
Ingore .dSYM/ directories

I have no idea what these are, but they're clearly a build product which
I don't want to see in my git status.
Ignore some exes in the junk/ directory
Add some junk code to explore calling dl* functions