Move declare_mgen_finite_alias definition

Move declare_mgen_finite_alias, declare_mgen_finite_alias_s and
declare_mgen_finite_alias_x to a shared place in order to reuse them in
other files that also declare _finite aliases.

	* math/e_exp2_template.c (declare_mgen_finite_alias,
	declare_mgen_finite_alias_s, declare_mgen_finite_alias_x): Move to...
	* sysdeps/generic/math-type-macros.h (declare_mgen_finite_alias,
	declare_mgen_finite_alias_s, declare_mgen_finite_alias_x): ... here.

Signed-off-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>