Add weak symbols as an extension to a.out.

	Add weak symbols as an extension to a.out.
	* read.c (pseudo_set): Only preserve external bit for OBJ_AOUT and
	OBJ_BOUT if not BFD_ASSEMBLER.
	* config/aout_gnu.h (N_WEAKU, N_WEAKA, N_WEAKT, N_WEAKD, N_WEAKB):
	Define as in ../include/aout/aout64.h.
	* config/obj-aout.h (OBJ_SYMFIELD_TYPE): If not BFD_ASSEMBLER,
	define as char.
	(S_GET_WEAK, S_SET_WEAK): Define if not BFD_ASSEMBLER.
	* config/obj-aout.c (obj_pseudo_table): Add "weak".
	(obj_emit_symbols): Adjust type of weak symbols.
	(obj_aout_weak): New static function.