Ada, Darwin: Implement OSLock for Darwin [PR115305].

The generic Posix code does not match the layout of the pthread entities
in the Darwin _pthread_types.h.  So, let's make a Darwin-specific version
and use it.

	PR ada/115305

gcc/ada/ChangeLog:

	* Makefile.rtl: Use s-oslock__darwin instead of the Posix version.
	* libgnat/s-oslock__darwin.ads: New file.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>