arm: introduce ret_from_kernel_execve(), switch to generic kernel_execve()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/arch/arm/include/asm/unistd.h b/arch/arm/include/asm/unistd.h
index 0cab47d..2c9b7a8 100644
--- a/arch/arm/include/asm/unistd.h
+++ b/arch/arm/include/asm/unistd.h
@@ -469,6 +469,7 @@
 #define __ARCH_WANT_OLD_READDIR
 #define __ARCH_WANT_SYS_SOCKETCALL
 #endif
+#define __ARCH_WANT_KERNEL_EXECVE
 
 /*
  * "Conditional" syscalls