MdeModulePkg/VarCheckUefiLib: permit use by MM_STANDALONE modules

Permit CheckUefiLib to be used by MM_STANDALONE modules. Since this
library has a constructor, change the library's module type into
BASE so its constructor prototype is compatible with MM_STANDALONE
as well.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>