ArmPkg/PrePeiCore: adhere to architectural stack alignment requirement

ArmPkg/PrePeiCore: adhere to architectural stack alignment requirement

Instead of using fuzzy arithmetic with a hardcoded stack alignment value
of 0x4, use the symbolic constant CPU_STACK_ALIGNMENT (which is at least
8 bytes, btw) to round the temporary stack base and size.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19163 6f19259b-4bc3-4df7-8a09-765794883524