Add pe/x86_64 bigobj file format.

bfd/
	* peicode.h (pe_ILF_object_p): Adjust, as the version number
	has been read.
	(pe_bfd_object_p): Also read version number to detect ILF.
	* pe-x86_64.c (COFF_WITH_PE_BIGOBJ): Define.
	(x86_64pe_bigobj_vec): Define
	* coffcode.h (bfd_coff_backend_data): Add _bfd_coff_max_nscns field.
	(bfd_coff_max_nscns): New macro.
	(coff_compute_section_file_positions): Use unsigned int for
	target_index.  Compare with bfd_coff_max_nscns.
	(bfd_coff_std_swap_table, ticoff0_swap_table, ticoff1_swap_table):
	Set a value for _bfd_coff_max_nscns.
	(header_bigobj_classid): New constant.
	(coff_bigobj_swap_filehdr_in, coff_bigobj_swap_filehdr_out)
	(coff_bigobj_swap_sym_in, coff_bigobj_swap_sym_out)
	(coff_bigobj_swap_aux_in, coff_bigobj_swap_aux_out): New
	functions.
	(bigobj_swap_table): New table.
	* libcoff.h: Regenerate.
	* coff-sh.c (bfd_coff_small_swap_table): Likewise.
	* coff-alpha.c (alpha_ecoff_backend_data): Add value for
	_bfd_coff_max_nscns.
	* coff-mips.c (mips_ecoff_backend_data): Likewise.
	* coff-rs6000.c (bfd_xcoff_backend_data)
	(bfd_pmac_xcoff_backend_data): Likewise.
	* coff64-rs6000.c (bfd_xcoff_backend_data)
	(bfd_xcoff_aix5_backend_data): Likewise.
	* targets.c (x86_64pe_bigobj_vec): Declare.
	* configure.in (x86_64pe_bigobj_vec): New vector.
	* configure: Regenerate.
	* config.bfd: Add bigobj object format for Windows targets.

gas/
	* config/tc-i386.c (use_big_obj): Declare.
	(OPTION_MBIG_OBJ): Define.
	(md_longopts): Add -mbig-obj option.
	(md_parse_option): Handle it.
	(md_show_usage): Display help for this option.
	(i386_target_format): Use bigobj for x86-64 if -mbig-obj.
	* doc/c-i386.texi: Document the option.

gas/testsuite/
	* gas/pe/big-obj.d, gas/pe/big-obj.s: Add test.
	* gas/pe/pe.exp: Add test.

include/coff/
	* pe.h (struct external_ANON_OBJECT_HEADER_BIGOBJ): Declare.
	(FILHSZ_BIGOBJ): Define.
	(struct external_SYMBOL_EX): Declare.
	(SYMENT_BIGOBJ, SYMESZ_BIGOBJ): Define.
	(union external_AUX_SYMBOL_EX): Declare.
	(AUXENT_BIGOBJ, AUXESZ_BIGOBJ): Define.
	* internal.h (struct internal_filehdr): Change type
	of f_nscns.