Fix ARM NAN fraction bits.

Current ARM soft-float implementation is violating the RTABI
(http://infocenter.arm.com/help/topic/com.arm.doc.ihi0043d/IHI0043D_rtabi.pdf)
Section 4.1.1.1:

When not otherwise specified by IEEE 754, the result on an invalid
operation should be the quiet NaN bit pattern with only the most
significant bit of the significand set, and all other significand bits
zero.

This patch fixes it by setting _FP_NANFRAC_* to zero.

Ran make check test with -mfloat-abi=soft. No regression.

	* sysdeps/arm/soft-fp/sfp-machine.h (_FP_NANFRAC_S, _FP_NANFRAC_D)
	(_FP_NANFRAC_Q): Set to zero.