Remove compat hacks from libcxxrt's _Unwind_Exception

This reverts 9097e3cbcac4, which was in itself a revert of upstream
libcxxrt commits 88bdf6b290da ("Specify double-word alignment for ARM
unwind") and b96169641f79 ("Updated Itanium unwind"), and a
reapplication of our commit 3c4fd2463bb2 ("libcxxrt: add padding in
__cxa_allocate_* to fix alignment").

The editors/libreoffice port will be patched to be able to cope with the
standards-compliant alignment of _Unwind_Exception and consequently,
that of __cxa_exception. The layouts and sizes of these structures
should then be completely the same for libcxxrt, libunwind and
libc++abi.

PR: 262008
Reviewed by: emaste, jhb, theraven
Approved by: re (gjb)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D34488

(cherry picked from commit 72df847a94bccee245a3316e4f848482b9ac2ac2)
(cherry picked from commit 93779214f1c64c0713f7c900021e35d4e4e49697)