[PATCH 6/7] sin/cos slow paths: refactor duplicated code into dosin

Refactor duplicated code into do_sin.  Since all calls to do_sin use copysign to
set the sign of the result, move it inside do_sin.  Small inputs use a separate
polynomial, so move this into do_sin as well (the check is based on the more
conservative case when doing large range reduction, but could be relaxed).

	* sysdeps/ieee754/dbl-64/s_sin.c (do_sin): Use TAYLOR_SIN for small
	inputs.  Return correct sign.
	(do_sincos): Remove small input check before do_sin, let do_sin set
	the sign.
	(__sin): Likewise.
	(__cos): Likewise.