Partially revert libcxxrt changes to avoid _Unwind_Exception change

(Note I am also applying this to main and stable/13, to restore the old
libcxxrt ABI and to avoid having to maintain a compat library.)

After the recent cherry-picking of libcxxrt commits 0ee0dbfb0d26 and
d2b3fadf2db5, users reported that editors/libreoffice packages from the
official package builders did not start anymore. It turns out that the
combination of these commits subtly changes the ABI, requiring all
applications that depend on internal details of struct _Unwind_Exception
(available via unwind-arm.h and unwind-itanium.h) to be recompiled.

However, the FreeBSD package builders always use -RELEASE jails, so
these still use the old declaration of struct _Unwind_Exception, which
is not entirely compatible. In particular, LibreOffice uses this struct
in its internal "uno bridge" component, where it attempts to setup its
own exception handling mechanism.

To fix this incompatibility, go back to the old declarations of struct
_Unwind_Exception, and restore the LP64 specific workaround we had
in place before (which was to cope with yet another, older ABI bug).

Effectively, this reverts upstream libcxxrt commits 88bdf6b290da
("Specify double-word alignment for ARM unwind") and b96169641f79
("Updated Itanium unwind"), and reapplies our commit 3c4fd2463bb2
("libcxxrt: add padding in __cxa_allocate_* to fix alignment").

PR: 253840