Let fpclassify use the builtin when optimizing for size in C++ mode (bug 22146)

When optimization for size is on (-Os), fpclassify does not use the
type-generic __builtin_fpclassify builtin, instead it uses __MATH_TG.
However, when library support for float128 is available, __MATH_TG uses
__builtin_types_compatible_p, which is not available in C++ mode.

On the other hand, libstdc++ undefines (in cmath) many macros from
math.h, including fpclassify, so that it can provide its own functions.
However, during its configure tests, libstdc++ just tests for the
availability of the macros (it does not undefine them, nor does it
provide its own functions).

Finally, when libstdc++ is configured with optimization for size
enabled, its configure tests include math.h and get the definition of
fpclassify that uses __MATH_TG (and __builtin_types_compatible_p).
Since libstdc++ does not undefine the macros during its configure tests,
they fail.

This patch lets fpclassify use the builtin in C++ mode, even when
optimization for size is on.  This allows the configure test in
libstdc++ to work.

Tested for powerpc64le and x86_64.

	[BZ #22146]
	math/math.h: Let fpclassify use the builtin in C++ mode, even
	when optimazing for size.

(cherry picked from commit c5c4a626)