UefiCpuPkg/Include/MpInitLib.h: Add MpInitLibStartupAllCPUs API.

UefiCpuPkg/Include/MpInitLib.h: Add MpInitLibStartupAllCPUs API.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1973

Add new MpInitLibStartupAllCPUs API uses to start all processors
at the same time.

Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Chandana Kumar <chandana.c.kumar@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>