Simplify NAN definitions.

Simplify NAN definitions.

Similar to my patches for HUGE_VAL and INFINITY. this patch eliminates
the bits/nan.h headers.  __builtin_nanf ("") is used to define NAN for
GCC 3.3 and later; the fallback is (0.0f / 0.0f), which is a constant
expression for a quiet NaN of type float, but raises a spurious
"invalid" exception outside static initializers, which seems the best
that can be done purely in standard C.  Again, if anyone actually uses
a compiler with its own incompatible extension for producing a
constant quiet NaN, we can add compiler conditionals.

Tested for x86_64.

	*  math/math.h [__USE_ISOC99] (NAN): Define directly here.  Do not
	include <bits/nan.h>.
	* math/Makefile (headers): Remove bits/nan.h.
	* bits/nan.h: Remove.
	* sysdeps/ieee754/bits/nan.h: Likewise.
	* sysdeps/mips/bits/nan.h: Likewise.