[csproj] Update generator to handle platform-specific assemblies (#4847)

[csproj] Update generator to handle platform-specific assemblies (#4847)

After https://github.com/mono/mono/pull/4837 was merged the
csproj update bot started sending PRs which hardcoded net_4_x-linux
as the OutputPath.

Updated the csproj generator to detect this and use a variable
in the OutputPath instead. It'll autodetect based on the current OS
and can be overriden by passing /p:HostPlatform=foobar to msbuild.

Ideally we'd have configurations for each platform in the csproj since
in the future the file lists will be different but this is a first step.