[ARM] Add arm_clear_feature() helper macro

There is no reason why we should'nt have macro to clear VCPU feature
just like having macro to set VCPU feature.

Currently, arm_clear_feature() is not of much importance but it will
help us with VFP context switching for arm32 port.

Signed-off-by: Anup Patel <anup@brainfault.org>