Merge libcxxrt master 8049924686b8414d8e652cbd2a52c763b48e8456

Interesting fixes:
b3c73ba libelftc_dem_gnu3: Sync with elftoolchain r3877
7b2335c Mostly fix __cxa_demangle after #3

Reported by: arichardson
PR: 253226

(cherry picked from commit 0ee0dbfb0d26cf4bc37f24f12e76c7f532b0f368)

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
Differential Revision: https://reviews.freebsd.org/D28799

(cherry picked from commit d2b3fadf2db56131376a71b0597876b591a6aee4)