math: Use wordsize-64 version for isnan

math: Use wordsize-64 version for isnan

  - math.h will use compiler builtin for gcc 4.4 when built without
    -fsignaling-nans and the builtin is expanded inline for all
    support architectures.  As an example, there is no intra isnan
    call on libm for the architecture I checked, x86, arm, aarch64,
    and powerpc.

  - The resulting binary difference on 32 bits architecture is minimum
    for the non hotspot symbol.

  - It helps wordsize-64 architectures that use ldbl-opt.

  - It add some code simplification with reduction of duplicated
    implementations.

Checked on powerpc-linux-gnu (built without --with-cpu, with
--with-cpu=power4 and with --with-cpu=power5+ and --disable-multi-arch),
powerpc64-linux-gnu (built without --with-cp and with --with-cpu=power5+
and --disable-multi-arch).

	* sysdeps/ieee754/dbl-64/wordsize-64/s_isnan.c: Move to ...
	* sysdeps/ieee754/dbl-64/s_isnan.c: ... here and format code.

Reviewed-by: Gabriel F. T. Gomes <gabrielftg@linux.ibm.com>