Fix ARM fail of gap test

Fix ARM fail of gap test

ld-elf/gap test was failing due to the ARM backend attempting to output
arch symbols when ld -s (strip all symbols) is in force.  This patch
stops that happening and tidies the code a little.

	PR 17842
	* elflink.c (elf_link_output_sym): Assert elf_onesymtab set.
	(bfd_elf_final_link): Always create a symbol table when emit_relocs.
	Don't assign symtab file position unless symbols will be output.
	Merge blocks with condition in common.  Don't call
	elf_backend_output_arch_local_syms or elf_backend_output_arch_syms
	unless other symbols are output.  Move assignment of symtab_shndx
	file position.  Localize variable.