runtime: annotate `_environ` DLL storage, exclude in WinRT
`_environ` is meant to be DLL imported when linking against the C
runtime dynamically (`/MD` or `/MDd`). However, when building for
the Windows Runtime environment, we cannot support the use of `_environ`
and thus disable the support for that. `_WINRT_DLL` identifies the
combination of `/ZW` and `/LD` or `/LDd`. Windows Runtime builds cannot
be executables (and obviously not static libraries as they are not
executable), and thus we properly disable `ENVIRON` in all Windows
Runtime builds.
