Generate ghcversion.h with the top-level configure

This is, rather unintuitively, part of the goal of making the packages
that make of the GHC distribution more freestanding. `ghcversion.h` is
very simple, so we easily can move it out of the main build systems
(make and Hadrian). By doing so, the RTS becomes less of a special case
to those build systems as the header, already existing in the source
tree, appears like any other.

We could do this with the upcomming RTS configure, but it hardly matters
because there is nothing platform-specific here, it is just versioning
information like the other files the top-level configure can be
responsible for.