ARC: Allow conditional multiple inclusion of uapi/asm/unistd.h

Commit 97bc386f "ARC: Add guard macro to uapi/asm/unistd.h"
inhibited multiple inclusion of ARCH unistd.h. This however hosed the system
since Generic syscall table generator relies on it being included twice,
and in lack-of an empty table was emitted by C preprocessor.

Fix that by allowing one exception to rule for the special case (just
like Xtensa)

Suggested-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>