ld: Extract _bfd_elf_link_iterate_on_relocs

ld: Extract _bfd_elf_link_iterate_on_relocs

DT_RELR encodes consecutive R_*_RELATIVE relocations in GOT (the global
offset table) and data sections in a compact format:

https://groups.google.com/g/generic-abi/c/bX460iggiKg

On some targets, R_*_RELATIVE relocations are counted and the GOT offsets
are allocated when setting the dynamic section sizes after seeing all
relocations.  R_*_RELATIVE relocations are generated while relocating
sections after section layout has been finalized.

To prepare for DT_RELR implementation on these targets, extract
_bfd_elf_link_iterate_on_relocs from _bfd_elf_link_check_relocs so
that a backend can scan relocations in elf_backend_always_size_sections

For x86 targets, the old check_relocs is renamed to scan_relocs and a
new check_relocs is added to chek input sections and create dynamic
relocation sections so that they will be mapped to output sections.
scan_relocs is now called from elf_backend_always_size_sections.

Since relocations are scanned after __start, __stop, .startof. and
.sizeof. symbols have been finalized on x86, __[start|stop]_SECNAME for
--gc-sections -z start-stop-gc are now zero when all SECNAME sections
been garbage collected.  This is no need for elf_x86_start_stop_gc_p.

bfd/

	* elf-bfd.h (_bfd_elf_link_iterate_on_relocs): New.
	* elf32-i386.c (elf_i386_convert_load_reloc): Don't call
	elf_x86_start_stop_gc_p.
	(elf_i386_check_relocs): Renamed to ...
	(elf_i386_scan_relocs): This.  Don't call
	_bfd_elf_make_dynamic_reloc_section.
	(elf_i386_always_size_sections): New.
	(elf_backend_check_relocs): Removed.
	(elf_backend_always_size_sections): New.
	* elf64-x86-64.c (elf_x86_64_convert_load_reloc): Don't call
	elf_x86_start_stop_gc_p.
	(elf_x86_64_check_relocs): Renamed to ...
	(elf_x86_64_scan_relocs): This.  Don't call
	_bfd_elf_make_dynamic_reloc_section.
	(elf_x86_64_always_size_sections): New.
	(elf_backend_check_relocs): Removed.
	(elf_backend_always_size_sections): New.
	* elflink.c (elf_link_check_or_scan_relocs):
	New.  Extracted from _bfd_elf_link_check_relocs.
	(_bfd_elf_link_check_relocs): Call elf_link_check_or_scan_relocs.
	* elfxx-x86.c (_bfd_x86_elf_check_relocs): New.
	* elfxx-x86.h (X86_64_NEED_DYNAMIC_RELOC_TYPE_P): New.
	(I386_NEED_DYNAMIC_RELOC_TYPE_P): Likewise.
	(X86_NEED_DYNAMIC_RELOC_TYPE_P): Likewise.
	(_bfd_x86_elf_check_relocs): Likewise.
	(elf_backend_check_relocs): Likewise.
	(elf_backend_always_size_sections): Removed.
	(elf_x86_start_stop_gc_p): Likewise.

ld/

	* testsuite/ld-i386/pr27491-1a.d: Updated.
	* testsuite/ld-x86-64/pr27491-1a.d: Likewise.