Make build system: Put make generated include's in RTS distdirs

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

• After !6791 (closed), 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 (closed) we'll start "morally" moving ghcautonconf.h over, introducing an RTS configure script and temporary header of its AC_DEFINEs until the top-level configure script doesn't define any more.

Progress towards #17191 (closed)