The LLVM backend on amd64 can sometimes cause top window borders to not
render properly for some inscrutable reason. Use the self-hosted backend
on amd64 since it's working quite well since Zig 0.16.0.
Signed-off-by: Violet Purcell <vi@vimproved.dev>
Let river's build.zig determine the backend used (self-hosted on amd64,
LLVM on all others). If users really want to override this, they can use
ZBS_EXTRA_ARGS.
Signed-off-by: Violet Purcell <vimproved@inventati.org>
