Add Linux/x86_64 core support to i[3-7]86-*-linux

Add Linux/x86_64 core support to i[3-7]86-*-linux

	* configure.in: Set CORE_HEADER to hosts/x86-64linux.h for
	'i[3-7]86-*-linux-*' if x86_64-*linux is enabled.
	* configure: Regenerated.

	* elf64-x86-64.c (elf_x86_64_write_core_note): Replace type
	'prpsinfo_t' and 'prstatus_t' with 'prpsinfo64_t' and
	'prstatus64_t' respectively.

	* hosts/x86-64linux.h (a8_uint64_t): New.
	(user_regsx32_struct): Renamed to ...
	(user_regs64_struct): This.  Replace uint64_t with a8_uint64_t.
	(elf_gregx32_t): Renamed to ...
	(elf_greg64_t): This.  Replace uint64_t with a8_uint64_t.
	(ELF_NGREGX32): Removed.
	(ELF_NGREG64): New.
	(elf_gregx32_t): Removed.
	(elf_greg64_t): New.
	(struct prstatus64_timeval): New.
	(elf_prstatusx32): Replace elf_gregsetx32_t with elf_gregset64_t.
	(elf_prstatus64): New.
	(elf_prpsinfo64): New.
	(prstatus64_t, prpsinfo64_t): New typedef.