Only reserve the aligned memory instead of reserving the unaligned memory in PeiMemoryAllocationLib

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Gao, Liming <liming.gao@intel.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdePkg@15878 6f19259b-4bc3-4df7-8a09-765794883524