m68k: Wire up clone3() syscall

Wire up the clone3() syscall for m68k. The special entry point is done in
assembler as was done for clone() as well. This is needed because all
registers need to be saved. The C wrapper then calls the generic
sys_clone3() with the correct arguments.

Tested on A1200 using the simple test program from:

  https://lore.kernel.org/lkml/20190716130631.tohj4ub54md25dys@brauner.io/

Signed-off-by: Kars de Jong <jongk@linux-m68k.org>
Link: https://lore.kernel.org/r/20191124195225.31230-1-jongk@linux-m68k.org
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/kernel/entry.S

@@ -69,6 +69,13 @@ ENTRY(__sys_vfork)
 	lea     %sp@(24),%sp
 	rts
 
+ENTRY(__sys_clone3)
+	SAVE_SWITCH_STACK
+	pea	%sp@(SWITCH_STACK_SIZE)
+	jbsr	m68k_clone3
+	lea	%sp@(28),%sp
+	rts
+
 ENTRY(sys_sigreturn)
 	SAVE_SWITCH_STACK
 	movel	%sp,%sp@-		  | switch_stack pointer