Fix powerpc software sqrtf (bug 17967).

Fix powerpc software sqrtf (bug 17967).

Similarly to sqrt in
<https://sourceware.org/ml/libc-alpha/2015-02/msg00353.html>, the
powerpc sqrtf implementation for when _ARCH_PPCSQ is not defined also
relies on a * b + c being contracted into a fused multiply-add.
Although this contraction is not explicitly disabled for e_sqrtf.c, it
still seems appropriate to make the file explicit about its
requirements by using __builtin_fmaf; this patch does so.
Furthermore, it turns out that doing so fixes the observed inaccuracy
and missing exceptions (that is, that without explicit __builtin_fmaf
usage, it was not being compiled as intended).

Tested for powerpc32 (hard float).

	[BZ #17967]
	* sysdeps/powerpc/fpu/e_sqrtf.c (__slow_ieee754_sqrtf): Use
	__builtin_fmaf instead of relying on contraction of a * b + c.