ArmPkg: move ARM version of SetMemoryAttributes to ArmMmuLib

... where it belongs, since AARCH64 already keeps it there, and
non DXE users of ArmMmuLib (such as DxeIpl, for the non-executable
stack) may need its functionality as well.

While at it, rename SetMemoryAttributes to ArmSetMemoryAttributes,
and make any functions that are not exported STATIC. Also, replace
an explicit gBS->AllocatePages() call [which is DXE specific] with
MemoryAllocationLib::AllocatePages().

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