devel/llvm20: Link LLDB with libxml2 to allow an interop with openocd

LLDB auto detects libxml2 by default and unlike with CLANG it's useful
for openocd support so depend on it.

Remove a seemingly useless USES=gnome from CLANG that we've been
carrying forward by accident.

While here, update to lua 5.4.

PR: 268303
Submitted by: arrowd (libxml2)
Differential Revision: https://reviews.freebsd.org/D50715