Use libm_alias_double for dbl-64 logb.

Use libm_alias_double for dbl-64 logb.

This patch makes dbl-64 logb use libm_alias_double.  Both the dbl-64
and dbl-64/wordsize-64 versions are changed, and the ldbl-opt version
is removed.  Because of multiarch wrappers, the changed
implementations are made not to define aliases at all if __logb is
defined as a macro, as with other functions, so avoiding duplicate
compat symbols while allowing those wrappers to be simplified.

Tested for x86_64, and verified with build-many-glibcs.py that
installed stripped shared libraries are unchanged (except on alpha
where changes from using the wordsize-64 version are expected).

	* sysdeps/ieee754/dbl-64/s_logb.c: Include <libm-alias-double.h>.
	(logb): Define using libm_alias_double, only if [!__logb].
	* sysdeps/ieee754/dbl-64/wordsize-64/s_logb.c: Include
	<libm-alias-double.h>.
	(logb): Define using libm_alias_double, only if [!__logb].
	* sysdeps/ieee754/ldbl-opt/s_logb.c: Remove file.
	* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_logb-ppc32.c
	(weak_alias): Do not undefine and redefine.
	(strong_alias): Likewise.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_logb-ppc64.c
	(weak_alias): Likewise.
	(strong_alias): Likewise.