libstdc++: Make std::make_exception_ptr work with -fno-exceptions [PR85813]

libstdc++: Make std::make_exception_ptr work with -fno-exceptions [PR85813]

This allows std::make_exception_ptr to be used in a translation unit
compiled with -fno-exceptions. This works because the new implementation
added for PR 68297 doesn't need to throw or catch anything. The catch is
there to handle exceptions from the constructor of the exception object,
which we can assume won't happen in a -fno-exceptions TU and so use the
__catch macro instead. If the constructor does throw (because it's
defined in a different TU which was compiled with exceptions enabled)
then that exception will propagate to the make_exception_ptr caller.
That seems acceptable for a program that is trying to mix & match TUs
compiled with and without exceptions, and using types that throw when
constructed. That should be rare, and can't reasonably be expected to
have sensible behaviour.

This also enables the new implementation for targets that use a
non-standard calling convention for the exceptionDestructor callback
(specifically, mingw, which uses __thiscall). All we need to do is mark
the __dest_thunk function template with the right calling convention.

Finally, the useless no-op definition of make_exception_ptr (which is
only used if both RTTI and exceptions are disabled) is marked
always_inline, to ensure that the linker won't keep that definition and
discard the functional ones when both definitions of the function are
present in the link. An alternative would be to add the abi_tag
attribute to the useless definition, but making it always_inline should
work, and it's small enough to always be inlined reliably.

libstdc++-v3/ChangeLog:

	PR libstdc++/85813
	* libsupc++/exception_ptr.h (__dest_thunk): Add macro for
	destructor calling convention.
	(make_exception_ptr): Enable non-throwing implementation for
	-fno-exceptions and for non-standard calling conventions. Use
	always_inline attribute on the useless no-rtti no-exceptions
	definition.
	* testsuite/18_support/exception_ptr/64241.cc: Add -fno-rtti so
	the no-op implementation is still used.