IPA: Provide a mechanism to register static DTORs via cxa_atexit.

IPA: Provide a mechanism to register static DTORs via cxa_atexit.

For at least one target (Darwin) the platform convention is to
register static destructors (i.e. __attribute__((destructor)))
with __cxa_atexit rather than placing them into a list that is
run by some other mechanism.

This patch provides a target hook that allows a target to opt
into this and handling for the process in ipa_cdtor_merge ().

When the mode is enabled (dtors_from_cxa_atexit is set) we:

 * Generate new CTORs to register static destructors with
   __cxa_atexit and add them to the existing list of CTORs;
   we then process the revised CTORs list.

 * We sort the DTORs into priority and then TU order, this
   means that they are registered in that order with
   __cxa_atexit () and therefore will be run in the reverse
   order.

 * Likewise, CTORs are sorted into priority and then TU order,
   which means that they will run in that order.

This matches the behavior of using init/fini (or
mod_init_func/mod_term_func) sections.

This also fixes a bug where Fortran needs a DTOR to be run to
close IO.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
	PR fortran/102992

gcc/ChangeLog:

	* config/darwin.h (TARGET_DTORS_FROM_CXA_ATEXIT): New.
	* doc/tm.texi: Regenerated.
	* doc/tm.texi.in: Add TARGET_DTORS_FROM_CXA_ATEXIT hook.
	* ipa.c (cgraph_build_static_cdtor_1): Return the built
	function decl.
	(build_cxa_atexit_decl): New.
	(build_dso_handle_decl): New.
	(build_cxa_dtor_registrations): New.
	(compare_cdtor_tu_order): New.
	(build_cxa_atexit_fns): New.
	(ipa_cdtor_merge): If dtors_from_cxa_atexit is set,
	process the DTORs/CTORs accordingly.
	(pass_ipa_cdtor_merge::gate): Also run if
	dtors_from_cxa_atexit is set.
	* target.def (dtors_from_cxa_atexit): New hook.