Enable x32 for x86_64-*-elf*

This patch enables x32 for x86_64-*-elf* for embedded target and disables
rex tests since it uses '/' as prefix separator which is `\' for
x86_64-*-elf*.

bfd/

	* config.bfd (targ_selvecs): Add bfd_elf32_x86_64_vec for
	x86_64-*-elf*.

gas/testsuite/

	* gas/i386/rex.d: Skip x86_64-*-elf*.
	* gas/i386/ilp32/rex.d: Likewise.

ld/

	* configure.tgt (targ_extra_emuls): Adds elf32_x86_64 for
	x86_64-*-elf*.
	(targ_extra_libpath): Likewise.
	(tdir_elf_i386): Replace x86_64 with i386 for x86_64-*-elf*.
	* dwarf2read.c (setup_type_unit_groups): Fix comment.