Stop powerpc copysignl raising "invalid" for sNaN argument (bug 20718).

The powerpc (hard-float) implementations of copysignl, both 32-bit and
64-bit, raise spurious "invalid" exceptions when the first argument is
a signaling NaN.  copysign functions should never raise exceptions
even for signaling NaNs.

The problem is the use of an fcmpu instruction to test the sign of the
high part of the long double argument.  This patch fixes the functions
to use fsel instead (as used for fabsl following my fixes for a
similar bug there), or to examine the integer representation for older
32-bit processors without fsel.

Tested for powerpc64 and powerpc32 (configurations with and without
fsel used).

	[BZ #20718]
	* sysdeps/powerpc/powerpc32/fpu/s_copysignl.S (__copysignl): Do
	not use floating-point comparisons to test sign.
	* sysdeps/powerpc/powerpc64/fpu/s_copysignl.S (__copysignl):
	Likewise.