Darwin: Reset section names table at the end of compile.

For a single use (typical compile) this vector will be reclaimed
as GGC.  For JIT this is not sufficient since it does not reset
the pointer to NULL (and thus we think the the vector is already
allocated when a context is reused).

The clears the vector and sets the pointer to NULL at the end
of object output.

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

gcc/ChangeLog:

	* config/darwin.c (darwin_file_end): Reset and reclaim the
	section names table at the end of compile.