libstdc++: Make C++20s operator wrappers operator() static.

libstdc++: Make C++20s operator wrappers operator() static.

The operator() for function objects introduced in C++20 (e.g., std::identity,
std::compare_three_way, std::ranges::equal) is now defined as static.
Although static operator() is a C++23 feature, it is supported in C++20 by
both GCC and clang (since their support was added in clang-16).

This change is not user-observable, as all affected operators are template
functions. Taking the address of such an operator requires casting to a pointer
to member function with a specific signature. The exact signature is unspecified
per C++20 [member.functions] p2 (e.g. due to potential parameters with default
arguments).

libstdc++-v3/ChangeLog:

	* include/bits/ranges_cmp.h (std::identity::operator()):
	(ranges::equal_to:operator(), ranges::not_equal_to:operator())
	(ranges::greater::operator(), ranges::greater_equal::operator())
	(ranges::less::operator(), ranges::less_equal::operator()):
	Declare as static.
	* libsupc++/compare (std::compare_three_way::operator()):
	Declare as static.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Reviewed-by: Patrick Palka <ppalka@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>