MdeModulePkg/MdeModulePkg.dsc: add MM_STANDALONE FTW and variable modules

For build testing coverage, add the newly introduced MM_STANDALONE
versions of the fault tolerant write (FTW) and variable runtime
driver to MdeModulePkg.dsc. Note that the resulting binaries will
not be able to run, since they rely on dummy implementations of
MemoryAllocationLib and HobLib.

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>.