Compute sin(pi*x) without actually doing the pi*x multiplication.
sin_pi(x) is only called for x < 0 and |x| < 2**(p-1) where p is
the precision of x.  The new argument reduction is an optimization
compared to the old code, and it removes a chunk of dead code.
Accuracy tests in the intervals (-21,-20), (-20,-19), ... (-1,0)
show no differences between the old and new code.

Obtained from:	bde


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270893 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f