powerpc: switch to generic sys_execve()/kernel_execve()

the only non-obvious part is that current_pt_regs() is really needed
here - task_pt_regs() is NULL for kernel threads; it's OK for ptrace
uses (the thing task_pt_regs() is intended for), but not for us.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
index 6eb330a..e6be75f 100644
--- a/arch/powerpc/kernel/entry_32.S
+++ b/arch/powerpc/kernel/entry_32.S
@@ -446,6 +446,11 @@
 	li	r3,0
 	b	do_exit		# no return
 
+	.globl	__ret_from_kernel_execve
+__ret_from_kernel_execve:
+	addi	r1,r3,-STACK_FRAME_OVERHEAD
+	b	ret_from_syscall
+
 /* Traced system call support */
 syscall_dotrace:
 	SAVE_NVGPRS(r1)