UefiCpuPkg/CpuDxe: Put APs in wait for SIPI state at ExitBootServices

when gBS->ExitBootServices() is called, the APs should avoid to access
the unsafed buff datas which were allocated by boot services.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Chen Fan <chen.fan.fnst@cn.fujitsu.com>
Tested-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/UefiCpuPkg@16397 6f19259b-4bc3-4df7-8a09-765794883524