math: Use wordsize-64 version for s_logb

  - The resulting binary difference on 32 bits architecture is
    minimum.  On i686-linux-gnu (with architecture optimization
    routine removed) there is no different using logb benchtests

  - 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_logb.c: Move to ...
	* sysdeps/ieee754/dbl-64/s_logb.c: ... here.  Add work around for
	powerpc32 integer 0 converting to -0.

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