Use libm_alias_double for dbl-64 frexp.

This patch makes dbl-64 frexp use libm_alias_double.  Both the dbl-64
and dbl-64/wordsize-64 versions are changed; the ldbl-opt version is
made to define only the libc frexpl compat symbol, now the generic
code handles the libm compat symbol automatically.

Tested for x86_64, and verified with build-many-glibcs.py that
installed stripped shared libraries are unchanged by this patch.

	* sysdeps/ieee754/dbl-64/s_frexp.c: Include <libm-alias-double.h>.
	(frexp): Define using libm_alias_double.
	* sysdeps/ieee754/dbl-64/wordsize-64/s_frexp.c: Include
	<libm-alias-double.h>.
	(frexp): Define using libm_alias_double.
	* sysdeps/ieee754/ldbl-opt/s_frexp.c (frexpl): Only define libc
	compat symbol here.