MdeModulePkg/DxeCorePerformanceLib: use AllocatePeiAccessiblePages

MdeModulePkg/DxeCorePerformanceLib: use AllocatePeiAccessiblePages

Replace the call to and implementation of the function
FpdtAllocateReservedMemoryBelow4G() with a call to
AllocatePeiAccessiblePages, which boils down to the same on X64,
but does not crash non-X64 systems that lack memory below 4 GB.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>