selftests/powerpc: fix exec benchmark

The exec_target binary could segfault calling _exit(2) because r13
is not set up properly (and libc looks at that when performing a
syscall). Call SYS_exit using syscall(2) which doesn't seem to
have this problem.

Signed-off-by: Nicholas Piggin <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>

tools/testing/selftests/powerpc/benchmarks/exec_target.c

···
  * Copyright 2018, Anton Blanchard, IBM Corp.
  */
 
-void _exit(int);
+#define _GNU_SOURCE
+#include <unistd.h>
+#include <sys/syscall.h>
+
 void _start(void)
 {
-	_exit(0);
+	syscall(SYS_exit, 0);
 }