SystemTools: Teach GetEnv/PutEnv to use correct encoding on Windows

On Windows getenv (and putenv) uses ANSI codepage so it needs to be encoded
to internally used encoding (eg. UTF-8). Here we use _wgetenv (and _wputenv)
instead and encode that.

Change-Id: I8cb91f2386eb0efe3ef0a3132d1603217d710b60