ANDROID: GKI: kernel: Export symbol of `cpu_do_idle`

lpm-levels module call this function which is located in
`arch/arm64/mm/proc.S`.

Export function so the module can be built.

Bug: 150365591
Bug: 149799837
Change-Id: Iff72ecbef7ac43a13909eaef896b5c50ce433650
Signed-off-by: lucaswei <lucaswei@google.com>
(cherry picked from commit d965ac77cb6e800d866dbf4f9be90d16a6b8f1e0)
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 141888626