Remove declare_mgen_alias_2.

Remove declare_mgen_alias_2.

The libm template machinery includes a macro declare_mgen_alias_2, to
define two function aliases rather than one.  This macro is only used
in one place, to define ldexp and scalbn, and only has one nondefault
definition, for double in the case where long double has the same
format.  That definition is because declare_mgen_alias for double, in
that case, defines <internal-func>l as an alias of <internal-func>, so
cannot be called twice for aliases of the same function.

Now, I suspect the <internal-func>l aliases are generally not needed
(with maybe a few exceptions such as __clog10l, which is an exported
function).  But even in the presence of them, there is no need for a
special declare_mgen_alias_2 macro for this case.  This patch
eliminates the need for such a macro by defining __wrap_scalbn<suffix>
as an alias of __ldexp<suffix>, and then using that when defining the
scalbn public aliases.  This is similar to how such internal aliases
are created for functions with multiple symbol versions, for example.

Tested for x86_64, and with build-many-glibcs.py.  (There *are* some
cases where installed stripped shared libraries change - not in the
generated code but because such changes to static symbols on input to
ld, even nonexported symbols that don't affect the code or dynamic
symbols, can affect the particular representation in the output of
string tables, hash tables etc.)

	* sysdeps/generic/math-type-macros.h [!declare_mgen_alias_2]
	(declare_mgen_alias_2): Remove.
	* sysdeps/generic/math-type-macros-double.h
	[NO_LONG_DOUBLE && !declare_mgen_alias_2] (declare_mgen_alias_2):
	Likewise.
	* math/s_ldexp_template.c (M_SUF (__wrap_scalbn)): Define strong
	alias.
	(ldexp): Define with declare_mgen_alias.
	(scalbn): Likewise.