Fix implib test failures

Fix implib test failures

bfd/
	* elf.c (_bfd_elf_filter_global_symbols): Skip local symbols.
	(swap_out_syms): Return an error when not finding ELF output
	section rather than asserting.
	* elflink.c (elf_output_implib): Call bfd_set_error on no symbols.
ld/
	* testsuite/lib/ld-lib.exp (run_ld_link_tests): Add optional
	parameter to pass list of xfails.
	* testsuite/ld-elf/elf.exp: Add xfails for implib tests.  Tidy
	implib test formatting.  Don't set .data start address.
	* testsuite/ld-elf/implib.s: Remove first .bss directive and
	replace second one with equivalent .section directive.
	* testsuite/ld-elf/empty-implib.out: Add expected final error.
	* testsuite/ld-elf/implib.rd: Update.