d: Implement __traits(getTargetInfo, "objectFormat")

Following on from adding TARGET_D_REGISTER_OS_TARGET_INFO, this adds the
required handlers to implement `__traits(getTargetInfo, "objectFormat")'
for all platforms that have D support files.

Some back-ends (i386, rs6000, and pa) have some awarenes of the what
object format they are compiling for, so new getTargetInfo handlers have
been have added both to those back-ends as well as platform-specific
target files to override the default in the D front-end.

gcc/ChangeLog:

	* config/darwin-d.c (darwin_d_handle_target_object_format): New
	function.
	(darwin_d_register_target_info): New function.
	(TARGET_D_REGISTER_OS_TARGET_INFO): Define.
	* config/dragonfly-d.c (dragonfly_d_handle_target_object_format): New
	function.
	(dragonfly_d_register_target_info): New function.
	(TARGET_D_REGISTER_OS_TARGET_INFO): Define.
	* config/freebsd-d.c (freebsd_d_handle_target_object_format): New
	function.
	(freebsd_d_register_target_info): New function.
	(TARGET_D_REGISTER_OS_TARGET_INFO): Define.
	* config/glibc-d.c (glibc_d_handle_target_object_format): New
	function.
	(glibc_d_register_target_info): New function.
	(TARGET_D_REGISTER_OS_TARGET_INFO): Define.
	* config/i386/i386-d.c (ix86_d_handle_target_object_format): New
	function.
	(ix86_d_register_target_info): Add ix86_d_handle_target_object_format
	as handler for objectFormat key.
	* config/i386/winnt-d.c (winnt_d_handle_target_object_format): New
	function.
	(winnt_d_register_target_info): New function.
	(TARGET_D_REGISTER_OS_TARGET_INFO): Define.
	* config/netbsd-d.c (netbsd_d_handle_target_object_format): New
	function.
	(netbsd_d_register_target_info): New function.
	(TARGET_D_REGISTER_OS_TARGET_INFO): Define.
	* config/openbsd-d.c (openbsd_d_handle_target_object_format): New
	function.
	(openbsd_d_register_target_info): New function.
	(TARGET_D_REGISTER_OS_TARGET_INFO): Define.
	* config/pa/pa-d.c (pa_d_handle_target_object_format): New function.
	(pa_d_register_target_info): Add pa_d_handle_target_object_format as
	handler for objectFormat key.
	* config/rs6000/rs6000-d.c (rs6000_d_handle_target_object_format): New
	function.
	(rs6000_d_register_target_info): Add
	rs6000_d_handle_target_object_format as handler for objectFormat key.
	* config/sol2-d.c (solaris_d_handle_target_object_format): New
	function.
	(solaris_d_register_target_info): New function.
	(TARGET_D_REGISTER_OS_TARGET_INFO): Define.

gcc/d/ChangeLog:

	* d-target.cc (d_handle_target_object_format): New function.
	(d_language_target_info): Add d_handle_target_object_format as handler
	for objectFormat key.
	(Target::getTargetInfo): Continue if handler returned NULL_TREE.