UefiCpuPkg/MpInitLib: Enable x2APIC mode on BSP/APs

UefiCpuPkg/MpInitLib: Enable x2APIC mode on BSP/APs

If x2APIC flag is set, enable x2APIC mode on all APs and BSP. Before we wakeup
APs to enable x2APIC mode, we should wait all APs have finished initialization.

Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Feng Tian <feng.tian@intel.com>
Cc: Giri P Mudusuru <giri.p.mudusuru@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Michael Kinney <michael.d.kinney@intel.com>