Add a TODO FIXME w.r.t. D894

Authored by austin.

Description

Add a TODO FIXME w.r.t. D894

As Reid mentioned in a comment on D894, the case fixed by this revision
likely isn't really correct, because old ARM binaries could run on newer
machines, meaning we need to detect at runtime whether we need a proper
barrier.

But in the mean time, this actually stops the build from failing - which
is better off. So we'll just remember this when we fix it in the future.

Signed-off-by: Austin Seipp <austin@well-typed.com>