libstdc++: Fix up std::{,b}float16_t std::{ilogb,l{,l}r{ound,int}} [PR117406]

These overloads incorrectly cast the result of the float __builtin_*
to _Float or __gnu_cxx::__bfloat16_t.  For std::ilogb that changes
behavior for the INT_MAX return because that isn't representable in
either of the floating point formats, for the others it is I think
just a very inefficient hop from int/long/long long to std::{,b}float16_t
and back.  I mean for the round/rint cases, either the argument is small
and then the return value should be representable in the floating point
format too, or it is too large that the argument is already integral
and then it should just return the argument with the round trips.
Too large value is unspecified unlike ilogb.

2024-11-02  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/117406
	* include/c_global/cmath (std::ilogb(_Float16), std::llrint(_Float16),
	std::llround(_Float16), std::lrint(_Float16), std::lround(_Float16)):
	Don't cast __builtin_* return to _Float16.
	(std::ilogb(__gnu_cxx::__bfloat16_t),
	std::llrint(__gnu_cxx::__bfloat16_t),
	std::llround(__gnu_cxx::__bfloat16_t),
	std::lrint(__gnu_cxx::__bfloat16_t),
	std::lround(__gnu_cxx::__bfloat16_t)): Don't cast __builtin_* return to
	__gnu_cxx::__bfloat16_t.
	* testsuite/26_numerics/headers/cmath/117406.cc: New test.

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