Mark internal _dl_XXX functions hidden

Since internal _dl_XXX functions are only used internally in ld.so,
they can be made hidden.

	[BZ #19122]
	* elf/dl-runtime.c (_dl_fixup): Add attribute_hidden.
	* elf/dynamic-link.h (_dl_try_allocate_static_tls): Likewise.
	* sysdeps/generic/dl-cache.h (_dl_cache_libcmp): Likewise.
	* sysdeps/generic/ldsodefs.h (_dl_name_match_p): Likewise.
	(_dl_higher_prime_number): Likewise.
	(_dl_debug_printf_c): Likewise.
	(_dl_signal_cerror): Likewise.
	(_dl_receive_error): Likewise.
	(_dl_reloc_bad_type): Likewise.
	(_dl_resolve_conflicts): Likewise.
	(_dl_check_all_versions): Likewise.
	(_dl_check_map_versions): Likewise.
	(_dl_sort_fini): Likewise.
	(_dl_debug_initialize): Likewise.
	(_dl_init_paths): Likewise.
	(_dl_show_auxv): Likewise.
	(_dl_next_ld_env_entry): Likewise.
	(_dl_important_hwcaps): Likewise.
	(_dl_load_cache_lookup): Likewise.
	(_dl_update_slotinfo): Likewise.
	(_dl_show_scope): Likewise.