ArmPkg/ArmDmaLib: use DMA buffer alignment from CPU arch protocol

ArmPkg/ArmDmaLib: use DMA buffer alignment from CPU arch protocol

Instead of depending on ArmLib to retrieve the CWG directly, use
the DMA buffer alignment exposed by the CPU arch protocol. This
removes our dependency on ArmLib, which makes the library a bit
more architecture independent.

While we're in there, rename gCpu to mCpu to better reflect its
local scope, and reflow some lines that we're modifying anyway.

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