x86-64: Add -z mark-plt and -z nomark-plt

x86-64: Add -z mark-plt and -z nomark-plt

The PLT entry in executables and shared libraries contains an indirect
branch, like

 	jmp *foo@GOTPCREL(%rip)
	push $index_foo
	jmp .PLT0

or

	endbr64
 	jmp *foo@GOTPCREL(%rip)
 	NOP padding

which is used to branch to the function, foo, defined in another object.
Each R_X86_64_JUMP_SLOT relocation has a corresponding PLT entry.

The dynamic tags have been added to the x86-64 psABI to mark such PLT
entries:

https://gitlab.com/x86-psABIs/x86-64-ABI/-/commit/6d824a52a42d173eb838b879616c1be5870b593e

Add an x86-64 linker option, -z mark-plt, to mark PLT entries with

 #define DT_X86_64_PLT     (DT_LOPROC + 0)
 #define DT_X86_64_PLTSZ   (DT_LOPROC + 1)
 #define DT_X86_64_PLTENT  (DT_LOPROC + 3)

1. DT_X86_64_PLT: The address of the procedure linkage table.
2. DT_X86_64_PLTSZ: The total size, in bytes, of the procedure linkage
table.
3. DT_X86_64_PLTENT: The size, in bytes, of a procedure linkage table
entry.

and set the r_addend field of the R_X86_64_JUMP_SLOT relocation to the
memory offset of the indirect branch instruction.  The dynamic linker
can use these tags to update the PLT section to direct branch.

bfd/

	* elf-linker-x86.h (elf_linker_x86_params): Add mark_plt.
	* elf64-x86-64.c (elf_x86_64_finish_dynamic_symbol): Set the
	r_addend of R_X86_64_JUMP_SLOT to the indirect branch offset
	in PLT entry for -z mark-plt.
	* elfxx-x86.c (_bfd_x86_elf_size_dynamic_sections): Add
	DT_X86_64_PLT, DT_X86_64_PLTSZ and DT_X86_64_PLTENT for
	-z mark-plt.
	(_bfd_x86_elf_finish_dynamic_sections): Set DT_X86_64_PLT,
	DT_X86_64_PLTSZ and DT_X86_64_PLTENT.
	(_bfd_x86_elf_get_synthetic_symtab): Ignore addend for
	JUMP_SLOT relocation.
	(_bfd_x86_elf_link_setup_gnu_properties): Set
	plt_indirect_branch_offset.
	* elfxx-x86.h (elf_x86_plt_layout): Add plt_indirect_branch_offset.

binutils/

	* readelf.c (get_x86_64_dynamic_type): New function.
	(get_dynamic_type): Call get_x86_64_dynamic_type.

include/

	* elf/x86-64.h (DT_X86_64_PLT): New.
	(DT_X86_64_PLTSZ): Likewise.
	(DT_X86_64_PLTENT): Likewise.

ld/

	* ld.texi: Document -z mark-plt and -z nomark-plt.
	* emulparams/elf32_x86_64.sh: Source x86-64-plt.sh.
	* emulparams/elf_x86_64.sh: Likewise.
	* emulparams/x86-64-plt.sh: New file.
	* testsuite/ld-x86-64/mark-plt-1.s: Likewise.
	* testsuite/ld-x86-64/mark-plt-1a-x32.d: Likewise.
	* testsuite/ld-x86-64/mark-plt-1a.d: Likewise.
	* testsuite/ld-x86-64/mark-plt-1b-x32.d: Likewise.
	* testsuite/ld-x86-64/mark-plt-1b.d: Likewise.
	* testsuite/ld-x86-64/mark-plt-1c-x32.d: Likewise.
	* testsuite/ld-x86-64/mark-plt-1c.d: Likewise.
	* testsuite/ld-x86-64/mark-plt-1d-x32.d: Likewise.
	* testsuite/ld-x86-64/mark-plt-1d.d: Likewise.
	* testsuite/ld-x86-64/x86-64.exp: Run -z mark-plt tests.