Clean up bits/math-finite.h for aliasing types.

This patch cleans up how bits/math-finite.h handles types that are
ABI-aliases of other types.

For such types, no __*_finite functions exist; instead,
bits/math-finite.h must redirect calls to a the functions for a
canonical choice of type for each floating-point format.  (For the
actual public interfaces, symbols need exporting for each type, even
those that are ABI-aliases, because of standard requirements that
programs can declare the functions themselves without including
<math.h>, but that does not apply to __*_finite.)

At present, there is a special-case conditional in bits/math-finite.h
on __MATH_DECLARING_LDOUBLE && defined __NO_LONG_DOUBLE_MATH to handle
redirecting long double function calls to double __*_finite.  This
patch replaces this by a more general mechanism.  math.h, before each
inclusion of bits/math-finite.h, defines _MSUFTO_ as the suffix to use
on the target of redirection, in addition to the existing _MSUF_.
This way, __MATH_DECLARING_LDOUBLE can go away, as can the special
conditional in bits/math-finite.h.  With this patch, math.h is now
prepared for the case of supporting float128 functions as aliases of
long double ones on platforms where long double is binary128, with
_MSUFTO_ appropriately defined for that case, and appropriate _MSUFTO_
definitions can easily be included when supporting _Float32 / _Float64
/ _Float32x / _Float64x (which will always be ABI-aliases of another
type when supported).

Tested for x86_64, and did a compilation test for ARM with
build-many-glibcs.py to cover the long double = double case.

	* math/math.h (_MSUFTO_): Define and undefine for each inclusion
	of <bits/math-finite.h>.
	(__MATH_DECLARING_LDOUBLE): Do not define and undefine for each
	inclusion of <bits/math-finite.h>.
	* math/bits/math-finite.h (__REDIRTO_X): Do not define
	conditionally on [__MATH_DECLARING_LDOUBLE && defined
	__NO_LONG_DOUBLE_MATH].
	(__MATH_REDIRCALL): Use _MSUFTO_ in __REDIRTO call.
	(__MATH_REDIRCALL_2): Likewise.
	(__MATH_REDIRCALL_INTERNAL): Likewise.
	(__REDIRFROM (lgamma, , _MSUF_)): Likewise.
	(__REDIRFROM (gamma, , _MSUF_)): Likewise.
	(__REDIRFROM (tgamma, , _MSUF_)): Likewise.

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