acpitests: fix build failure for multilib

bitbake lib32-acpitests
[snip]
| NOTE: make -j 2  -m32 OPT_CFLAGS=-Wall
| make: *** No rule to make target ` -m32'.  Stop.
| ERROR: oe_runmake failed
[snip]

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>