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.
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