static-PIE: Update DT_DEBUG for debugger [BZ #23206]

This is needed to support debugging dlopened shared libraries in static
PIE.

	[BZ #23206]
	* elf/dl-reloc-static-pie.c (_dl_relocate_static_pie): Initialize
	_r_debug and update DT_DEBUG for debugger.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>