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: NChen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: NVineet Gupta <vgupta@synopsys.com>

arch/arc/include/uapi/asm/unistd.h

@@ -8,7 +8,11 @@
 
 /******** no-legacy-syscalls-ABI *******/
 
-#ifndef _UAPI_ASM_ARC_UNISTD_H
+/*
+ * Non-typical guard macro to enable inclusion twice in ARCH sys.c
+ * That is how the Generic syscall wrapper generator works
+ */
+#if !defined(_UAPI_ASM_ARC_UNISTD_H) || defined(__SYSCALL)
 #define _UAPI_ASM_ARC_UNISTD_H
 
 #define __ARCH_WANT_SYS_EXECVE
@@ -36,4 +40,6 @@ __SYSCALL(__NR_arc_gettls, sys_arc_gettls)
 #define __NR_sysfs		(__NR_arch_specific_syscall + 3)
 __SYSCALL(__NR_sysfs, sys_sysfs)
 
+#undef __SYSCALL
+
 #endif