ARC: fix build warnings with !CONFIG_KPROBES

|   CC      lib/nmi_backtrace.o
| In file included from ../include/linux/kprobes.h:43:0,
|                  from ../lib/nmi_backtrace.c:17:
| ../arch/arc/include/asm/kprobes.h:57:13: warning: 'trap_is_kprobe' defined but not used [-Wunused-function]
|  static void trap_is_kprobe(unsigned long address, struct pt_regs *regs)
|              ^~~~~~~~~~~~~~

The warning started with 7d134b2c ("kprobes: move kprobe declarations
to asm-generic/kprobes.h") which started including <asm/kprobes.h>
unconditionally into <linux/kprobes.h> exposing a stub function for
!CONFIG_KPROBES to rest of world. Fix that by making the stub a macro
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/include/asm/kprobes.h

@@ -54,9 +54,7 @@ int kprobe_fault_handler(struct pt_regs *regs, unsigned long cause);
 void kretprobe_trampoline(void);
 void trap_is_kprobe(unsigned long address, struct pt_regs *regs);
 #else
-static void trap_is_kprobe(unsigned long address, struct pt_regs *regs)
-{
-}
+#define trap_is_kprobe(address, regs)
 #endif /* CONFIG_KPROBES */
 
 #endif /* _ARC_KPROBES_H */