arch: arc: enable US bit to check user mode

Enable us bit to check user mode more efficienly.

US is read as zero in user mode. This will allow use mode sleep
instructions, and it enables a form of denial-of-service attack
by putting the processor in sleep mode, but since interrupt
level/mask can't be set from user space that's not worse than
executing a loop without yielding.

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>