Add support for creating ELF import libraries

Add support for creating ELF import libraries

2016-07-15  Thomas Preud'homme  <thomas.preudhomme@arm.com>

bfd/
	* elf-bfd.h (elf_backend_filter_implib_symbols): Declare backend hook.
	(_bfd_elf_filter_global_symbols): Declare.
	* elf.c (_bfd_elf_filter_global_symbols): New function.
	* elflink.c (elf_filter_global_symbols): Likewise.
	(elf_output_implib): Likewise.
	(bfd_elf_final_link): Call above function, failing if it does.
	* elfxx-target.h (elf_backend_filter_implib_symbols): Define macro and
	default it to NULL.
	(elf_backend_copy_indirect_symbol): Fix spacing.
	(elf_backend_hide_symbol): Likewise.
	(elfNN_bed): Initialize elf_backend_filter_implib_symbols backend hook.

include/
	* bfdlink.h (struct bfd_link_info): Declare new ldscript_def and
	out_implib_bfd fields.

2016-07-15  Thomas Preud'homme  <thomas.preudhomme@arm.com>
	    Nick Clifton  <nickc@redhat.com>

ld/
	* emultempl/elf32.em (gld${EMULATION_NAME}_after_open): Open import
	library file for writing and initialize implib_bfd field of link_info
	structure.
	* emultempl/pe.em (pe_implib_filename): Remove variable declaration.
	(OPTION_IMPLIB_FILENAME): Remove macro definition.
	(gld${EMULATION_NAME}_add_options): Remove --out-implib option.
	(gld_${EMULATION_NAME}_list_options): Likewise.
	(gld${EMULATION_NAME}_handle_option): Likewise.
	(gld_${EMULATION_NAME}_finish): Use command_line.out_implib_filename
	instead of pe_implib_filename.
	* emultempl/pep.em (pep_implib_filename): Remove variable declaration.
	(OPTION_IMPLIB_FILENAME): Remove enumerator.
	(gld${EMULATION_NAME}_add_options): Remove --out-implib option.
	(gld_${EMULATION_NAME}_list_options): Likewise.
	(gld${EMULATION_NAME}_handle_option): Likewise.
	(gld_${EMULATION_NAME}_finish): Use command_line.out_implib_filename
	instead of pep_implib_filename.
	* ld.h (args_type): Declare new out_implib_filename field.
	* ld.texinfo (--out-implib): Move documentation to arch-independent
	part and rephrase to apply to ELF targets.
	* ldexp.c (exp_fold_tree_1): Set ldscript_def field to 1 for symbols
	defined in linker scripts.
	* ldlex.h (enum option_values): Declare new OPTION_OUT_IMPLIB
	enumerator.
	* lexsup.c (ld_options): Add entry for new --out-implib switch.
	(parse_args): Handle OPTION_OUT_IMPLIB case.
	* testsuite/ld-elf/elf.exp (Generate empty import library): New test.
	(Generate import library): Likewise.
	* testsuite/ld-elf/implib.s: Likewise.
	* testsuite/ld-elf/implib.rd: New file.
	* testsuite/ld-elf/empty-implib.out: Likewise