x86: entry_64.S - use ENTRY to define child_rip

child_rip is called not by its name but indirectly
rather so make it global and aligned.
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Acked-by: Alexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/entry_64.S

@@ -1160,7 +1160,7 @@ ENTRY(kernel_thread)
 	CFI_ENDPROC
 END(kernel_thread)
 
-child_rip:
+ENTRY(child_rip)
 	pushq $0		# fake return address
 	CFI_STARTPROC
 	/*