Unable to build cabal with some older GHC after spec-field-cabalspecversion merges

Hi,

I have seen since the `spec-field-cabalspecversion` commits were merged that I can no longer build Cabal using GHC `8.4.3` which seem to cause compiler panics.

```
cabal: Failed to build Cabal-3.3.0.0 (which is required by test:unit-tests
from Cabal-3.3.0.0, test:rpmvercmp from Cabal-3.3.0.0 and others).
ghc.exe: panic! (the 'impossible' happened)
(GHC version 8.4.3 for x86_64-unknown-mingw32):
Prelude.!!: index too large

Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
```
I haven't taken a look at the changes to see if it's cased by TH or an actual compiler issue, I can simply update my build compiler.. but is it intentional that Cabal no longer supports building on older GHCs?

/CC @phadej