gas: utility notes memory alloc functions

Makes it a little easier to use the notes obstack for persistent
storage.

	* as.h (gas_mul_overflow): Define.
	* symbols.h (notes_alloc, notes_calloc, notes_memdup),
	(notes_strdup, notes_concat, notes_free): Declare.
	* symbols.c (notes_alloc, notes_calloc, notes_memdup),
	(notes_strdup, notes_concat, notes_free): New functions.
	(save_symbol_name): Use notes_strdup.
	(symbol_create, local_symbol_make, local_symbol_convert),
	(symbol_clone, decode_local_label_name): Use notes_alloc.