EmbeddedPkg/PrePiLib: allocate code pages for DxeCore

EmbeddedPkg/PrePiLib: allocate code pages for DxeCore

The recently introduced memory protection features inadvertently broke
the boot on all PrePi platforms, because the changes to explicitly use
EfiBootServicesCode for loading the DxeCore PE/COFF image need to be
applied in a different way for PrePi. So add a simple helper function
that sets the type of an allocation to EfiBootServicesCode, and invoke
it to allocate the space for DxeCore.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Michael Zimmermann <sigmaepsilon92@gmail.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>