patches: Backport upstream fix for __is_convertible

patches: Backport upstream fix for __is_convertible

Replicate upstream commit 484e64f7e7b2c0494d7b2dbfdd528bcd707ee652.
libcxx previously used the `__is_convertible_to` builtin which wasn't
supported by GCC; this patch adds support for `__is_convertible` which
is available as of GCC 13.

Signed-off-by: Andrei Tatar <andrei@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Maria Sfiraiala <maria.sfiraiala@gmail.com>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #28