c++: use __cxa_call_terminate for MUST_NOT_THROW [PR97720]

[except.handle]/7 says that when we enter std::terminate due to a throw,
that is considered an active handler.  We already implemented that properly
for the case of not finding a handler (__cxa_throw calls __cxa_begin_catch
before std::terminate) and the case of finding a callsite with no landing
pad (the personality function calls __cxa_call_terminate which calls
__cxa_begin_catch), but for the case of a throw in a try/catch in a noexcept
function, we were emitting a cleanup that calls std::terminate directly
without ever calling __cxa_begin_catch to handle the exception.

A straightforward way to fix this seems to be calling __cxa_call_terminate
instead.  However, that requires exporting it from libstdc++, which we have
not previously done.  Despite the name, it isn't actually part of the ABI
standard.  Nor is __cxa_call_unexpected, as far as I can tell, but that one
is also used by clang.  For this case they use __clang_call_terminate; it
seems reasonable to me for us to stick with __cxa_call_terminate.

I also change __cxa_call_terminate to take void* for simplicity in the front
end (and consistency with __cxa_call_unexpected) but that isn't necessary if
it's undesirable for some reason.

This patch does not fix the issue that representing the noexcept as a
cleanup is wrong, and confuses the handler search; since it looks like a
cleanup in the EH tables, the unwinder keeps looking until it finds the
catch in main(), which it should never have gotten to.  Without the
try/catch in main, the unwinder would reach the end of the stack and say no
handler was found.  The noexcept is a handler, and should be treated as one,
as it is when the landing pad is omitted.

The best fix for that issue seems to me to be to represent an
ERT_MUST_NOT_THROW after an ERT_TRY in an action list as though it were an
ERT_ALLOWED_EXCEPTIONS (since indeed it is an exception-specification).  The
actual code generation shouldn't need to change (apart from the change made
by this patch), only the action table entry.

	PR c++/97720

gcc/cp/ChangeLog:

	* cp-tree.h (enum cp_tree_index): Add CPTI_CALL_TERMINATE_FN.
	(call_terminate_fn): New macro.
	* cp-gimplify.cc (gimplify_must_not_throw_expr): Use it.
	* except.cc (init_exception_processing): Set it.
	(cp_protect_cleanup_actions): Return it.

gcc/ChangeLog:

	* tree-eh.cc (lower_resx): Pass the exception pointer to the
	failure_decl.
	* except.h: Tweak comment.

libstdc++-v3/ChangeLog:

	* libsupc++/eh_call.cc (__cxa_call_terminate): Take void*.
	* config/abi/pre/gnu.ver: Add it.

gcc/testsuite/ChangeLog:

	* g++.dg/eh/terminate2.C: New test.