powerpc/kgdb: Do not set kgdb_single_step on ppc

The kgdb_single_step flag has the possibility to indefinitely
hang the system on an SMP system.

The x86 arch have the same problem, and that problem was fixed by
commit 8097551d(kgdb,x86: do not set kgdb_single_step
on x86). This patch does the same behaviors as x86's patch.
Signed-off-by: Dongdong Deng <dongdong.deng@windriver.com>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/kgdb.c

@@ -410,7 +410,6 @@ int kgdb_arch_handle_exception(int vector, int signo, int err_code,
 #else
 			linux_regs->msr |= MSR_SE;
 #endif
-			kgdb_single_step = 1;
 			atomic_set(&kgdb_cpu_doing_single_step,
 				   raw_smp_processor_id());
 		}