alpha: add clone3() support

Since clone3() needs the full register state saved for copying into
the child, it needs the same kind of wrapper as fork(), vfork() and
clone().  Exact same wrapper works, actually...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Author: Al Viro

arch/alpha/kernel/entry.S

@@ -811,6 +811,7 @@ alpha_\name:
 fork_like fork
 fork_like vfork
 fork_like clone
+fork_like clone3
 
 .macro	sigreturn_like name
 	.align	4

arch/alpha/kernel/syscalls/syscall.tbl

@@ -474,7 +474,7 @@
 542	common	fsmount				sys_fsmount
 543	common	fspick				sys_fspick
 544	common	pidfd_open			sys_pidfd_open
-# 545 reserved for clone3
+545	common	clone3				alpha_clone3
 546	common	close_range			sys_close_range
 547	common	openat2				sys_openat2
 548	common	pidfd_getfd			sys_pidfd_getfd