ArmPlatformPkg: use correct ASM decoration for non-function global symbols

ArmPlatformPkg: use correct ASM decoration for non-function global symbols

This fixes the declaration and definition of mSystemMemoryEnd so that it
is correctly annotated as a non-function symbol. Also adds the ASM_PFX
prefix, which is empty on AARCH64 but should be included for correctness.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17860 6f19259b-4bc3-4df7-8a09-765794883524