Fixes https://github.com/mono/mono/issues/10602 .
From the issue:
```
We need to enable this to support the system assemblies conflict
resolution which we now rely on for any new packages to enhance
developers experience and get us out of dependency on specific package
versions.
```
* [msbuild] Set $(CscDebugFileExt) also, whenever overriding $(CscToolExe)
- This property was introduced in mono's msbuild, but will be upstream
also
- This *must* be set before Microsoft.CSharp.targets file is imported.
- Even though the msbuild targets will automatically select `.mdb` if
the `$(CscToolExe)` is `mcs` or `mcs.exe`, it would be a good
practice to set both the properties together.
- This came up in cases where we use `smcs`, because in that case the
msbuild targets end up using `.pdb`.
* [msbuild] Set `$(CscDebugFileExt)` == `.mdb`
Xamarin.ObjcBinding.CSharp.targets: Set the debug file extension also,
since we are overriding the compiler via `$(CscToolExe)`. Also, move
the property definition around to ensure that they are set *before*
importing `Microsoft.CSharp.targets`.
Ankit Jain
Rolf Bjarne Kvinge
Jeffrey Stedfast