UefiCpuPkg RegisterCpuFeaturesLib: Fix buffer pointer error usage.

UefiCpuPkg RegisterCpuFeaturesLib: Fix buffer pointer error usage.

Current code allocate buffer for the pointer which later get value
from PCD database. but current code error use "=" for this case.
Use AllocateCopyPool instead to fix it.

V2 enhanced to directly use AllocateCopyPool to get the PCD value.
V3 enhanced to avoid using local temp variable.
V4 enhanced to keep the functions to get the pcd values.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Shao Ming <ming.shao@intel.com>
Cc: Kinney Michael D <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Kinney Michael D <michael.d.kinney@intel.com>