elf/x86-64: Subtract __ImageBase for R_AMD64_IMAGEBASE

elf/x86-64: Subtract __ImageBase for R_AMD64_IMAGEBASE

When linking Windows x86-64 relocatable object files to generate x86-64
ELF executable, we need to subtract __ImageBase, aka __executable_start,
for R_AMD64_IMAGEBASE relocation:

1. Add link_info to struct output_elf_obj_tdata to store linker info and
_bfd_get_link_info() to retrieve it.
2. Add ldelf_set_output_arch to set up link_info.
3. Add pex64_link_add_symbols to create an indirect reference to
__executable_start for __ImageBase to support R_AMD64_IMAGEBASE relocation
when adding symbols from Windows x86-64 relocatable object files to
generate x86-64 ELF executable.
4. Also subtract __ImageBase for R_AMD64_IMAGEBASE when generating x86-64
ELF executable.

bfd/

	PR ld/27425
	PR ld/27432
	* bfd.c (_bfd_get_link_info): New function.
	* elf-bfd.h (output_elf_obj_tdata): Add link_info.
	(elf_link_info): New.
	* libbfd-in.h (_bfd_get_link_info): New prototype.
	* coff-x86_64.c (coff_amd64_reloc): Also subtract __ImageBase for
	R_AMD64_IMAGEBASE when generating x86-64 ELF executable.
	* pe-x86_64.c: Include "coff/internal.h" and "libcoff.h".
	(pex64_link_add_symbols): New function.
	(coff_bfd_link_add_symbols): New macro.
	* libbfd.h: Regenerated.

ld/

	PR ld/27425
	PR ld/27432
	* ldelf.c (ldelf_set_output_arch): New function.
	* ldelf.h (ldelf_set_output_arch): New prototype.
	* emultempl/elf.em (LDEMUL_SET_OUTPUT_ARCH): Default to
	ldelf_set_output_arch.
	* ld-x86-64/pe-x86-64-1.od: Expect __executable_start.
	* testsuite/ld-x86-64/pe-x86-64-2.od: Likewise.
	* testsuite/ld-x86-64/pe-x86-64-3.od: Likewise.
	* testsuite/ld-x86-64/pe-x86-64-4.od: Likewise.
	* testsuite/ld-x86-64/pe-x86-64-5.od: Likewise.
	* testsuite/ld-x86-64/pe-x86-64-5.rd: Likewise.
	* testsuite/ld-x86-64/pe-x86-64-6.obj.bz2: New file.
	* testsuite/ld-x86-64/pe-x86-64-6.od: Likewise.
	* testsuite/ld-x86-64/pe-x86-64.exp: Run ld/27425 test.