UefiCpuPkg/CpuDxe: StartupAllAPs in parallel mode

UefiCpuPkg/CpuDxe: StartupAllAPs in parallel mode

SetMemoryAttributes() will sync BSP's MTRRs settings to all APs by StartupAllAPs
service in serial mode. It may caused much performance impact if there are too
much processors in system. This update is to invoke StartupAllAps in parallel
mode. IA32 SDM does suggest to program MTRRs in parallel mode.

Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Feng Tian <feng.tian@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: Feng Tian <feng.tian@intel.com>