gas: replace bfd_alloc with notes_alloc

bfd_alloc can return NULL on out-of-memory so code needs to check the
return value and print an error.  That check was missing in write.c.
notes_alloc won't return NULL, instead the underlying obstack_alloc
prints an OOM message and the process exits.  This is more convenient,
and when the bfd_alloc memory is attached to the gas output bfd it is
released only slightly before the notes obstack.

	* config/obj-macho.c (obj_mach_o_set_indirect_symbols): Use
	notes_calloc rather than bfd_zalloc.
	* write.c (set_symtab): Use notes_alloc.