UefiCpuPkg/MpInitLib: Avoid ApInitReconfig in PEI.

UefiCpuPkg/MpInitLib: Avoid ApInitReconfig in PEI.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2683

In PEI phase, AP already been waked up through ApInitConfig,
so it can directly wake up it through change wakup buffer
instead of use ApInitReconfig flag. It can save some time.

Change code to only use ApInitReconfig flag in DXE phase
which must need to update the wake up buffer.

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