d: Add TARGET_D_TEMPLATES_ALWAYS_COMDAT

Following up on the fix for PR99914, when testing on MinGW, it was found
not to support weak in the same way as on ELF or Mach-O targets.

So the linkage has been reverted back to COMDAT for that target, however
in order to properly support overriding functions and variables, all
declarations with external linkage must be put on COMDAT.  For this a
new target hook has been added to control the behavior.

gcc/ChangeLog:

	PR d/99914
	* config/i386/winnt-d.c (TARGET_D_TEMPLATES_ALWAYS_COMDAT): Define.
	* doc/tm.texi: Regenerate.
	* doc/tm.texi.in (D language and ABI): Add @hook for
	TARGET_D_TEMPLATES_ALWAYS_COMDAT.

gcc/d/ChangeLog:

	PR d/99914
	* d-target.def (d_templates_always_comdat): New hook.
	* d-tree.h (mark_needed): Remove prototype.
	* decl.cc: Include d-target.h.
	(mark_needed): Rename to...
	(d_mark_needed): ...this.  Make static.
	(set_linkage_for_decl): Put variables in comdat if
	d_templates_always_comdat.