Fix powerpc software sqrt (bug 17964).

As Adhemerval noted in
<https://sourceware.org/ml/libc-alpha/2015-01/msg00451.html>, the
powerpc sqrt implementation for when _ARCH_PPCSQ is not defined is
inaccurate in some cases.

The problem is that this code relies on fused multiply-add, and relies
on the compiler contracting a * b + c to get a fused operation.  But
sysdeps/ieee754/dbl-64/Makefile disables contraction for e_sqrt.c,
because the implementation in that directory relies on *not* having
contracted operations.

While it would be possible to arrange makefiles so that an earlier
sysdeps directory can disable the setting in
sysdeps/ieee754/dbl-64/Makefile, it seems a lot cleaner to make the
dependence on fused operations explicit in the .c file.  GCC 4.6
introduced support for __builtin_fma on powerpc and other
architectures with such instructions, so we can rely on that; this
patch duly makes the code use __builtin_fma for all such fused
operations.

Tested for powerpc32 (hard float).

2015-02-12  Joseph Myers  <joseph@codesourcery.com>

	[BZ #17964]
	* sysdeps/powerpc/fpu/e_sqrt.c (__slow_ieee754_sqrt): Use
	__builtin_fma instead of relying on contraction of a * b + c.

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