Simplify //mojo/public/c/system targets

We don't support building mojo apps in the component build, so we can
simplify these system targets a bit. We don't need to have everything
depend on a forwarding group that always evaluates to the same thing in
the mojo repo.

The new targets are:

//mojo/public/c/system which contains the system headers but no
implementation.

//mojo/public/platform/native:system which contains the system thunks
and exported thunk setter, used by anything run by the shell. This is
mostly used by the mojo_native_application template but is also used
by a few targets that need to link as shared_lib but will ultimately
be loaded by the shell.

//mojo/edk/system provides the real implementation of the system calls
and is used by the shell and a few tests.

The :for_... groups need to stick around for now to keep chromium
working, but once this rolls in to the chromium tree then these can
be fixed up as well.

R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/754963008