[xbuild] Fix mscorlib resolution for custom frameworks

Now that mcs supports multi-targeting, we can use the MSBuild 4.0+
system of finding mscorlib.dll in the target framework folders and
passing it to the compiler, along with the /nostdlib argument.

Like MSBuild, the old NoStdLib property can be set to true to disable
passing mscorlib.dll to the compiler, and the new NoCompilerStandardLib
property can be set to false to disable passing /nostdlib to the compiler.

This makes custom target frameworks work correctly without having to
override CscExe with a wrapper script.