Support C compilation with an empty package db

Summary

Currently GHC doesn't support compiling C code with an empty package db. That's because it looks up the ghcversion.h file in the RTS package and raises an error if there is none. We should support this. It is particularly useful to bootstrap the RTS.

Steps to reproduce

> touch foo.c
> ghc -c foo.c -clear-package-db
ghc-9.12.2: ghcversion.h missing; tried:

Expected behavior

It should just work.

Environment

GHC versions used: 8.4 to 9.12 at least

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