UefiCpuPkg/MpInitLib: fix AP init issue in 64-bit PEI

UefiCpuPkg/MpInitLib: fix AP init issue in 64-bit PEI

This issue is introduced by a patch at

   f32bfe6d061420a15bac6083063d227c567e6388

The above patch miss the case of 64-bit PEI, which will link
X64/MpFuncs.nasm instead of Ia32/MpFuncs.nasm. For X64/MpFuncs.nasm,
ExchangeInfo->ModeHighMemory should be always initialized no matter
if separate wakeup buffer is allocated or not. Ia32/MpFuncs.nasm will
not need ModeHighMemory during AP init. So the changes made in this
patch should not affect the functionality of it.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>