gas: switch convert_to_bignum() to taking just an expression

gas: switch convert_to_bignum() to taking just an expression

Both callers, despite spelling things differently, now pass the same
input for its 2nd parameter. Therefore, as was supposed to be the case
anyway, this 2nd parameter isn't needed anymore - the function can
calculate "sign" all by itself from the incoming expression. Instead
make the function return the resulting value, for emit_expr_with_reloc()
to consume for setting its "extra_digit" local variable.