arc: Replace lkml.org links with lore

As started by commit 05a5f51c ("Documentation: Replace lkml.org
links with lore"), replace lkml.org links with lore to better use a
single source that's more likely to stay available long-term.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>