[System] Remove internal Win32* classes from mobile builds

[System] Remove internal Win32* classes from mobile builds

This reduce the size of System.dll and also removes some Win32 specific
p/invokes.

They cause native linking issues for tvOS as Xcode 7.1 does not allow
`-u` in bitcode-enabled builds so we have to call directly the symbols
and it must exists at (native) link time.