ArmPkg/ArmBaseLib: clean up directory structure

ArmPkg/ArmBaseLib: clean up directory structure

For historical reasons, the files under ArmLib are split up into 'common'
files under Common/, containing common C files as well as AArch64 and Arm
specific asm files, and ArmV7 and AArch64 files under ArmV7/ and AArch64/,
respectively. This presumably dates back to the time when ArmLib supported
different revisions of the 32-bit architecture (i.e., pre-V7)

Since the PI spec requires V7 or later, we can simplify this to Arm/ and
AArch64, which aligns ArmLib with the majority of other modules that carry
ARM or AArch64 specific code.

So move the files around so that shared files live at the same level as
ArmBaseLib.inf, and ARM/AArch64 specific files live in Arm/ or AArch64/,
respectively.

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