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>