UefiCpuPkg/PeiMpLib: Fix a system hang-in-pei issue.

GetWakeupBuffer() tries to find a below-1M free memory, it checks
whether the memory is allocated already in
CheckOverlapWithAllocatedBuffer(). When there is a memory allocation
hob (base = 0xff_00000000, size = 0x10000000),
CheckOverlapWithAllocateBuffer() truncates the base to 0 which causes
it always returns TRUE so GetWakeupBuffer() fails to find a below-1MB
memory.

The patch fixes this issue by using UINT64 type.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jeff Fan <vanjeff_919@hotmail.com>