* read.c (convert_to_bignum): New function, split out from...

	* read.c (convert_to_bignum): New function, split out from...
	(emit_expr): ...here.  Handle the case where X_add_number is
	positive and the input value is negative.
	(output_big_sleb128): Fix setting of continuation bit.  Check whether
	the final byte needs to be sign-extended.  Fix size-shrinking loop.
	(emit_leb128_expr): When generating a signed leb128, see whether the
	sign of an O_constant's X_add_number matches the sign of the input
	value.  Use a bignum if not.