function signbit( x ) {
	return Boolean( addon( x ) );
}
Returns a boolean indicating if the sign bit is on (true) or off (false).

@private
@param {number} x - input value
@returns {boolean} - boolean indicating if sign bit is on or off

@example
var bool = signbit( 4.0 );
// returns false

@example
var bool = signbit( -9.14e-307 );
// returns true

@example
var bool = signbit( 0.0 );
// returns false

@example
var bool = signbit( -0.0 );
// returns true