Fix GHC_STAGE definition generated by make

Fixes #18070 (closed)

GHC_STAGE is the stage of the compiler we're building, it should be 1,2(,3?). But make was generating 0 and 1.

Hadrian does this correctly using a similar + 1: https://gitlab.haskell.org/ghc/ghc/-/blob/eb8115a8c4cbc842b66798480fefc7ab64d31931/hadrian/src/Rules/Generate.hs#L245

Fix due to @leonschoorl