Add 'stm' package to the global package database

This is a preparation for `haskeline` picking up a dependency on `stm`
real soon now. See https://github.com/judah/haskeline/pull/61 for details.

If we figure out a way to not bundle the libraries depended upon by the
GHCi executable in the global package database (see #8919 for the original
reason why we had to start bundling terminfo/haskeline in the first place)
we can get rid of `stm` again...

On the bright side, we were able to avoid uploading new `stm` releases for
over two years already, so it shouldn't cause too much trouble if GHC imposes
a strong preference on the `stm` package's version (this most likely will
mostly affect Linux distributions & similiar).

While at it, this also update the stm submodule to include relaxed
bounds to allow the upcoming base-4.11 version.