Previously, ARC .config's would get built for this test, giving Kconfig
warnings along the lines of
warning: NUM_IRQS ... was assigned the value "3" but got the value
"38". Check its dependencies...
Add an 'arch_whitelist: arm' so that .configs only get built for ARM.
These warnings will soon be turned into errors.
Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>