`__GLASGOW_HASKELL_LLVM__` is not defined even if `-fllvm` is set
## Summary
[The docs](https://downloads.haskell.org/ghc/9.12.1/docs/users_guide/phases.html) say `__GLASGOW_HASKELL_LLVM__` is defined if `-fllvm` is set. This is not the case.
## Steps to reproduce
```
$ cat test_llvm.hs
{-# LANGUAGE CPP #-}
main :: IO ()
main = do
#if defined(__GLASGOW_HASKELL_LLVM__)
putStrLn $ "__GLASGOW_HASKELL_LLVM__ = " ++ show __GLASGOW_HASKELL_LLVM__
#else
putStrLn "__GLASGOW_HASKELL_LLVM__ not defined"
#endif
$ grep LLVM ~/.ghcup/ghc/9.12.1/lib/ghc-9.12.1/lib/settings
,("LLVM target", "arm64-apple-darwin")
,("LLVM llc command", "/opt/homebrew/opt/llvm@18/bin/llc")
,("LLVM opt command", "/opt/homebrew/opt/llvm@18/bin/opt")
,("LLVM llvm-as command", "clang")
$ ghc-9.12.1 -fllvm -fforce-recomp test_llvm.hs
$ ./test_llvm
__GLASGOW_HASKELL_LLVM__ not defined
```
## Expected behavior
The program above should print `__GLASGOW_HASKELL_LLVM__ = 1800` or something.
## Environment
* GHC version used: 9.12.1
* Operating System: Ubuntu (with LLVM from APT) / macOS (with LLVM from Homebrew)
* System Architecture: x86_64 / AArch64
issue