Revert 3c4fd2463bb2 since upstream libcxxrt fixed it in another way

Revert 3c4fd2463bb2 since upstream libcxxrt fixed it in another way

In 0ee0dbfb0d26cf4bc37f24f12e76c7f532b0f368 I imported a more recent
libcxxrt snapshot, which includes an upstream fix for the padding of
struct _Unwind_Exception:

https://github.com/libcxxrt/libcxxrt/commit/e458560b7e22fff59af643dba363544b393bd8db

However, we also had a similar fix in our tree as:
https://cgit.freebsd.org/src/commit/?id=3c4fd2463bb29f65ef1404011fcb31e508cdf2e2

Since having both fixes makes the struct too large again, it leads to
SIGBUSes when throwing exceptions on amd64 (or other LP64 arches). This
is most easily tested by running kyua without any arguments.

It looks like our fix is no longer needed now, so revert it to reduce
diffs against upstream.

PR:		253226
Reviewed by:	arichardson, kp
MFC after:	3 days
Differential Revision: https://reviews.freebsd.org/D28799