ArmVirtPkg: adapt ArmVirtXen build to system memory end global variable

ArmVirtPkg: adapt ArmVirtXen build to system memory end global variable

This fixes the ArmVirtXen build that was broken by r17835, which adds
a global variable mSystemMemoryEnd which is shared between a module
and a library it depends on.

Add the same global variable to the relocatable PrePi used by ArmVirtXen.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17837 6f19259b-4bc3-4df7-8a09-765794883524