UefiCpuPkg CpuExceptionHandlerLib: use FixedPcdGetSize() as the macro value

FixedPcdGetSize() is used as the macro value, PcdGetSize() is used as global
variable or function. Here usage is to access macro value.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Liming Gao <liming.gao@intel.com>
Cc: Wang Jian J <jian.j.wang@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>