John Ericson authored Oct 24, 2021 and Marge Bot committed Oct 31, 2021

These are best thought of as being part of the RTS.

 - After !6791, `ghcautoconf.h` won't be used by the compiler
   inappropriately.

 - `ghcversion.h` is only used once outside the RTS, which is
   `compiler/cbits/genSym.c`. Except we *do* mean the RTS GHC is built
   against there, so it's better if we always get get the installed
   version.

 - `ghcplatform.h` alone is used extensively outside the RTS, but
   since we no longer have a target platform it is perfectly
   safe/correct to get the info from the previous RTS.

All 3 are exported from the RTS currently and in the bootstrap window.

This commit just swaps directories around, such that the new headers may
continue to be used in stage 0 despite the reasoning above, but the idea
is that we can subsequently make more interesting changes doubling down
on the reasoning above.

In particular, in !6803 we'll start "morally" moving `ghcautonconf.h`
over, introducing an RTS configure script and temporary header of its
`AC_DEFINE`s until the top-level configure script doesn't define any
more.

Progress towards #17191