UefiCpuPkg/MpInitLib: Get ApLoopMode and MointorFilter size

UefiCpuPkg/MpInitLib: Get ApLoopMode and MointorFilter size

Firstly, get ApLoopMode from PcdCpuApLoopMode. If MonitorMwait feature is not
supported, update ApLoopMode to ApHltLoop. If MonitorMwait feature is supported,
get MointorFilter size by CPUID.[EAX=05H]:EBX.BIT0-15.

v5:
  1. Add comment block for enum AP_LOOP_MODE.

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>