arm64: vdso: Cleanup Makefiles

The recent changes to the vdso library for arm64 and the introduction of
the compat vdso library have generated some misalignment in the
Makefiles.

Cleanup the Makefiles for vdso and vdso32 libraries:
  * Removing unused rules.
  * Unifying the displayed compilation messages.
  * Simplifying the generic library inclusion path for
    arm64 vdso.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>