New obstack_next_free is not an lvalue

New obstack_next_free is not an lvalue

New obstack.h casts obstack_next_free to (void *), resulting in it
being a non-lvalue, and warnings on pointer arithmetic.

gcc/
	* gensupport.c (add_mnemonic_string): Make len param a size_t.
	(gen_mnemonic_setattr): Make "size" var a size_t.  Use
	obstack_blank_fast to shrink obstack.  Cast obstack_next_free
	return value.
gcc/objc/
	* objc-encoding.c (encode_aggregate_within): Cast obstack_next_free
	return value.

From-SVN: r229984