Use libm_alias_double for dbl-64 modf.

Use libm_alias_double for dbl-64 modf.

This patch makes dbl-64 modf use libm_alias_double.  Both the dbl-64
and dbl-64/wordsize-64 versions are changed, and the ldbl-opt version
is changed to define the libc compat symbol only.  Because of
multiarch wrappers, the changed implementations are made not to define
aliases at all if __modf 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 by the patch.

	* sysdeps/ieee754/dbl-64/s_modf.c: Include <libm-alias-double.h>.
	(modf): Define using libm_alias_double, only if [!__modf].
	* sysdeps/ieee754/dbl-64/wordsize-64/s_modf.c: Include
	<libm-alias-double.h>.
	(modf): Define using libm_alias_double, only if [!__modf].
	* sysdeps/ieee754/ldbl-opt/s_modf.c (modfl): Only define libc
	compat symbol here.
	* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_modf-ppc32.c
	(weak_alias): Do not undefine and redefine.
	(strong_alias): Likewise.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_modf-ppc64.c
	(weak_alias): Likewise.
	(strong_alias): Likewise.