Fix GHC_STAGE definition generated by make

Fixes #18070

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
23 jobs for master in 3 minutes and 17 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Lint
passed #409899
lint
lint-linters

00:01:37

failed #409902
lint allowed to fail
lint-submods

00:00:13

failed #409903
lint
lint-submods-branch

00:00:22

passed #409900
lint
lint-testsuite

00:01:43

passed #409901
lint
typecheck-testsuite

00:00:15

 
  Quick Build
skipped #409904
x86_64-linux
hadrian-ghc-in-ghci
 
  Build
skipped #409905
x86_64-linux
validate-x86_64-linux-deb9-hadrian
 
  Full Build
skipped #409908
aarch64-linux allowed to fail
validate-aarch64-linux-deb9
skipped #409909
armv7-linux allowed to fail
validate-armv7-linux-deb9
skipped #409910
x86_64-linux
validate-i386-linux-deb9
skipped #409907
x86_64-darwin
validate-x86_64-darwin
skipped #409911
x86_64-linux
validate-x86_64-linux-deb9-debug
skipped #409913
x86_64-linux
validate-x86_64-linux-deb9-dwarf
skipped #409912
x86_64-linux
validate-x86_64-linux-deb9-integer-simple
skipped #409906
x86_64-linux
validate-x86_64-linux-deb9-unreg-hadrian
skipped #409914
x86_64-linux
validate-x86_64-linux-fedora27
skipped #409916
new-x86_64-windows test allowed to fail
validate-x86_64-windows
skipped #409915
new-x86_64-windows test allowed to fail
validate-x86_64-windows-hadrian
 
  Cleanup
passed #409917
x86_64-darwin
cleanup-darwin

00:00:59

 
  Packaging
skipped #409918
x86_64-linux allowed to fail
doc-tarball
 
  Testing
skipped #409919
x86_64-linux allowed to fail manual
hackage
skipped #409920
x86_64-linux
perf-nofib
 
  Deploy
skipped #409921
x86_64-linux allowed to fail
pages
 
Name Stage Failure
failed
lint-submods-branch Lint
Submodule update(s) detected in a625719284db7c69fa3d122e829291a16960e85f:
libraries/text => 80cb9ee2eb7141171171318bbd6760fe80012524
OK
Submodule update(s) detected in 12f9035200424ec8104484f154a040d612fee99d:
utils/haddock => 22b42eab6ec6b3b321b6d54041b7b3a6e54af3c9
OK
Submodule update(s) detected in e9e04ddae1bf89902803d86282f41a586620c58f:
*FAIL* commit message does not contain magic 'submodule' word
ERROR: Job failed: exit code 1
failed
lint-submods Lint
Entering 'libraries/unix'
Entering 'libraries/xhtml'
Entering 'nofib'
Entering 'utils/haddock'
Entering 'utils/hsc2hs'
Authenticating with credentials from job payload (GitLab Registry)
$ git fetch "$CI_MERGE_REQUEST_PROJECT_URL" $CI_MERGE_REQUEST_TARGET_BRANCH_NAME
fatal: No path specified. See 'man git-pull' for valid url syntax
ERROR: Job failed: exit code 1