MdeModulePkg/MdeModulePkg.dsc: ignore standalone MM modules for EBC or XCODE5

MdeModulePkg/MdeModulePkg.dsc: ignore standalone MM modules for EBC or XCODE5

The newly added standalone MM versions of the FTW and variable runtime
drivers were included in MdeModulePkg.dsc to get test coverage when
building the package from its own .dsc, but the resulting modules are
non-functional since they incorporate some dummy libraries.

Dandan reports that these modules don't build cleanly when using the
EBC or XCODE5 compilers, so given the above, let's just ignore them
in this case.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Bi Dandan <dandan.bi@intel.com>