libstdc++: Fix multiple definitions of std::exception_ptr functions [PR 97729]

This fixes some multiple definition errors caused by the changes for
PR libstdc++/90295. The previous solution for inlining the members of
std::exception_ptr but still exporting them from the library was to
suppress the 'inline' keyword on those functions when compiling
libsupc++/eh_ptr.cc, so they get defined in that file. That produces ODR
violations though, because there are now both inline and non-inline
definitions in the library, due to the use of std::exception_ptr in
other files sucg as src/c++11/future.cc.

The new solution is to define all the relevant members as 'inline'
unconditionally, but use __attribute__((used)) to cause definitions to
be emitted in libsupc++/eh_ptr.cc as before. This doesn't quite work
however, because PR c++/67453 means the attribute is ignored on
constructors and destructors. As a workaround, the old solution
(conditionally inline) is still used for those members, but they are
given the always_inline attribute so that they aren't emitted in
src/c++11/future.o as inline definitions.

libstdc++-v3/ChangeLog:

	PR libstdc++/97729
	* include/std/future (__basic_future::_M_get_result): Use
	nullptr for null pointer constant.
	* libsupc++/eh_ptr.cc (operator==, operator!=): Remove
	definitions.
	* libsupc++/exception_ptr.h (_GLIBCXX_EH_PTR_USED): Define
	macro to conditionally add __attribute__((__used__)).
	(operator==, operator!=, exception_ptr::exception_ptr())
	(exception_ptr::exception_ptr(const exception_ptr&))
	(exception_ptr::~exception_ptr())
	(exception_ptr::operator=(const exception_ptr&))
	(exception_ptr::swap(exception_ptr&)): Always define as
	inline. Add macro to be conditionally "used".

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