Support _Float64, _Float32x in libm_alias_double.

Support _Float64, _Float32x in libm_alias_double.

This patch makes the libm_alias_double macros support creating
_Float64 and _Float32x aliases, in preparation for enabling glibc
support for those types.

Tested for x86_64; also tested with build-many-glibcs.py in
conjunction with other _Float64 / _Float32x changes.

	* sysdeps/generic/libm-alias-double.h: Include <bits/floatn.h>.
	(libm_alias_double_other_r_f64): New macro.
	(libm_alias_double_other_r_f32x): Likewise.
	(libm_alias_double_other_r): Use libm_alias_double_other_r_f64 and
	libm_alias_double_other_r_f32x.
	(libm_alias_double_r): Use semicolon before call to
	libm_alias_double_other_r.
	* sysdeps/ieee754/ldbl-opt/libm-alias-double.h: Include
	<bits/floatn.h>.
	(libm_alias_double_other_r_f64): New macro.
	(libm_alias_double_other_r_f32x): Likewise.
	(libm_alias_double_other_r): Use libm_alias_double_other_r_f64 and
	libm_alias_double_other_r_f32x.