Смотри в /usr/src/sys/.../conf/LINT, если есть строка SMP OPTIONS, значит поддерживается.
Или еще 'man SMP' читаем:
<cat>
HISTORY
The SMP kernel's early history is not (properly) recorded. It was devel-
oped in a separate CVS branch until April 26, 1997, at which point it was
merged into 3.0-current. By this date 3.0-current had already been
merged with Lite2 kernel code.
</cat>
:)