Add extern_protected_data and set it for x86

Add extern_protected_data and set it for x86

With copy relocation, address of protected data defined in the shared
library may be external.  This patch adds extern_protected_data and
changes _bfd_elf_symbol_refs_local_p to return false for protected data
if extern_protected_data is true.

Backport from master:

bfd/

	PR ld/pr15228
	PR ld/pr17709
	* elf-bfd.h (elf_backend_data): Add extern_protected_data.
	* elf32-i386.c (elf_backend_extern_protected_data): New.
	Defined to 1.
	* elf64-x86-64.c (elf_backend_extern_protected_data): Likewise.
	* elflink.c (_bfd_elf_adjust_dynamic_copy): Don't error on
	copy relocs against protected symbols if extern_protected_data
	is true.
	(_bfd_elf_symbol_refs_local_p): Don't return true on protected
	non-function symbols if extern_protected_data is true.
	* elfxx-target.h (elf_backend_extern_protected_data): New.
	Default to 0.
	(elfNN_bed): Initialize extern_protected_data with
	elf_backend_extern_protected_data.

ld/testsuite/

	PR ld/pr15228
	PR ld/pr17709
	* ld-i386/i386.exp (i386tests): Add a test for PR ld/17709.
	* ld-i386/pr17709-nacl.rd: New file.
	* ld-i386/pr17709.rd: Likewise.
	* ld-i386/pr17709a.s: Likewise.
	* ld-i386/pr17709b.s: Likewise.
	* ld-i386/protected3.d: Updated.
	* ld-i386/protected3.s: Likewise.
	* ld-x86-64/pr17709-nacl.rd: New file.
	* ld-x86-64/pr17709.rd: Likewise.
	* ld-x86-64/pr17709a.s: Likewise.
	* ld-x86-64/pr17709b.s: Likewise.
	* ld-x86-64/protected3.d: Updated.
	* ld-x86-64/protected3.s: Likewise.
	* ld-x86-64/x86-64.exp (x86_64tests): Add a test for PR ld/17709.