c++: cross-module __cxa_atexit use [PR 98531]

c++: cross-module __cxa_atexit use [PR 98531]

The compiler's use of lazily-declared library functions must insert
said functions into a symbol table, so that they can be correctly
merged across TUs at the module-level.  We have too many different
ways of declaring such library functions.  This fixes __cxa_atexit (or
its system-specific variations), pushing (or merging) the decl into
the appropriate namespace.  Because we're pushing a lazy builtin,
check_redeclaration_exception_specification needed a tweak to allow a
such a builtin's eh spec to differ from what the user may have already
declared. (I suspect no all headers declare atexit as noexcept.)

We can't test the -fno-use-cxa-atexit path with modules, as that
requires a followup patch to a closely related piece (which also
affects cxa_atexit targets in other circumstances).

	PR c++/98531
	gcc/cp/
	* cp-tree.h (push_abi_namespace, pop_abi_namespace): Declare.
	* decl.c (push_abi_namespace, pop_abi_namespace): Moved
	from rtti.c, add default namespace arg.
	(check_redeclaration_exception_specification): Allow a lazy
	builtin's eh spec to differ from an lready-declared user
	declaration.
	(declare_global_var): Use push/pop_abi_namespace.
	(get_atexit_node): Push the fndecl into a namespace.
	* rtti.c (push_abi_namespace, pop_abi_namespace): Moved to
	decl.c.
	gcc/testsuite/
	* g++.dg/modules/pr98531-1.h: New.
	* g++.dg/modules/pr98531-1_a.H: New.
	* g++.dg/modules/pr98531-1_b.C: New.
	* g++.dg/abi/pr98531-1.C: New.
	* g++.dg/abi/pr98531-2.C: New.
	* g++.dg/abi/pr98531-3.C: New.
	* g++.dg/abi/pr98531-4.C: New.