Remove need for mono resgen

Remove need for mono resgen

The last remaining dependency on mono was to use the resgen
implementation since cross platform we don't have a version of
ResGen.exe. While MSBuild and dotnet-resgen can both convert txt files
into resources, neither of them understand the #if syntax we use in
mscorlib.txt to allow us to remove resources if features of the runtime
are not enabled.

Since we already have a dependency on Python, I've added a dead simple
script that understands the limited preprocessor features we support
and pre-processes the text file. We can then pass that preprocessed file
to MSBuild which will handle converting it to a .resources file.

[tfs-changeset: 1579180]

Commit migrated from https://github.com/dotnet/coreclr/commit/29f52c442a08240b3df1c5893901f9dfcae6751f