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>