ARC: fix build warnings

| arch/arc/mm/tlb.c:914:2: warning: variable length array 'pd0' is used [-Wvla]
| arch/arc/include/asm/cmpxchg.h:95:29: warning: value computed is not used [-Wunused-value]

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>