cmake.bbclass: fix allarch override syntax

The override syntax should be ":allarch" instead of "_allarch".

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>