Start trying to make the float precision trig functions actually worth

using under FreeBSD.  Before this commit, all float precision functions
except exp2f() were implemented using only float precision, apparently
because Cygnus needed this in 1993 for embedded systems with slow or
inefficient double precision.  For FreeBSD, except possibly on systems
that do floating point entirely in software (very old i386 and now
arm), this just gives a more complicated implementation, many bugs,
and usually worse performance for float precision than for double
precision.  The bugs and worse performance were particulary large in
arg reduction for trig functions.  We want to divide by an approximation
to pi/2 which has as many as 1584 bits, so we should use the widest
type that is efficient and/or easy to use, i.e., double.  Use fdlibm's
__kernel_rem_pio2() to do this as Sun apparently intended.  Cygnus's
k_rem_pio2f.c is now unused.  e_rem_pio2f.c still needs to be separate
from e_rem_pio2.c so that it can be optimized for float args.  Similarly
for long double precision.

This speeds up cosf(x) on large args by a factor of about 2.  Correct
arg reduction on large args is still inherently very slow, so hopefully
these args rarely occur in practice.  There is much more efficiency
to be gained by using double precision to speed up arg reduction on
medium and small float args.

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