Fix crossgenning of framework libraries on Windows (#747)

Fix crossgenning of framework libraries on Windows (#747)

1) Fix crossgenning of framework libraries on Windows;

2) Enable crossgenning S.P.C with CG2, remove obsolete CoreCLR logic;

3) Fix framework build with legacy Crossgen due to a typo in my change;

4) The ordering of steps in build-test was incorrect - we need to
patch CORE_ROOT with live-live libraries build before Crossgenning
the framework therein, otherwise weird things happen (and the result
is incorrect in any case).

5) Removed misplaced >nul 2>nul - we agreed with JanV that it's
better to not throw diagnostic information away.

6) Applied JanV's suggestion for improvement - using a tighter filter
for reference assemblies - Microsoft.*.dll / System.*.dll /
mscorlib.dll instead of just *.dll that ended up picking tons of
garbage like clrjit.dll and the various Win32 API contracts, both
in the build-test script and in CLRTest.Crossgen.targets.

Thanks

Tomas