Isolate the target-specific choice to 2 functions instead of 6.

The code in float16_default_nan was only correct for ARM, MIPS, and X86.
Though float16 support is rare among our targets.

The code in float128_default_nan was arguably wrong for Sparc.  While
QEMU supports the Sparc 128-bit insns, no real cpu enables it.

The code in floatx80_default_nan tried to be over-general.  There are
only two targets that support this format: x86 and m68k.  Thus there
is no point in inventing a value for snan_bit_is_one.

Move routines that no longer have ifdefs out of softfloat-specialize.h.
Tested-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>