Fix fallout from migration from alloca() to malloc() in the previous.

of_compatible() is used at least for macppc in very early boot stage
where malloc() is still not available.

Therefore, for small (<= OFW_MAX_STACK_BUF_SIZE = 256) buffer, use
statically allocated one in the stack. For larger one, we continue to
use malloc() but KASSERT(!cold) is added for sure.

Fix boot failure for macppc reported by martin:
http://mail-index.netbsd.org/port-macppc/2020/06/25/msg002756.html

(rin)