Add experimental support for --gc-sections with COFF and PE based targets.

Add experimental support for --gc-sections with COFF and PE based targets.

	PR ld/11539
bfd	* coffcode.h (coff_bfd_gc_sections): Define default
	to bfd_coff_gc_sections function.
	* cofflink.c (init_reloc_cookie): Copy and adjust coff
	related code about gc-sections from elflink.c to here.
	(fini_reloc_cookie): Likewise.
	(init_reloc_cookie_rels): Likewise.
	(fini_reloc_cookie_rels): Likewise.
	(init_reloc_cookie_for_section): Likewise.
	(fini_reloc_cookie_for_section): Likewise.
	(_bfd_coff_gc_mark_hook): Likewise.
	(_bfd_coff_gc_mark_rsec): Likewise.
	(_bfd_coff_gc_mark_reloc): Likewise.
	(_bfd_coff_gc_mark): Likewise.
	(_bfd_coff_gc_mark_extra_sections): Likewise.
	(coff_gc_sweep_symbol_info): Likewise.
	(coff_gc_sweep_symbol): Likewise.
	(gc_sweep_hook_fn): Likewise.
	(coff_gc_sweep): Likewise.
	(bfd_coff_gc_sections): Likewise.
	(_bfd_coff_gc_keep): Likewise.
	* libcoff.h (coff_reloc_cookie): New struct.
	(bfd_coff_gc_sections): New prototype.
	(coff_gc_mark_hook_fn): New type.

ld	* scripttempl/pep.sc: Mark .idata*, .CRT*, .tls*,
	.rsrc*, .init, .ctor*, .dtor*, .fini, .jcr,
	.eh_frame, .pdata. .xdata, and .gcc_except_table sections
	as KEEP.
	* scripttempl/pe.sc: Likewise.