haskell.compiler.ghcjs: Fix Linux build (#151699)

Before this change the GHCJS build fails on Linux with:

```
Setup: Missing dependency on a foreign library:
* Missing (or bad) header file: ghc/utils/unlit/fs.h
```

The root cause is that the `./ghc/configure.ac` script is
responsible for copying `./ghc/utils/fs/fs.h` to
`./ghc/utils/unlit/fs.h`, but the script exits early if a C
compiler is not present.

This fixes that by ensuring that the C compiler is present
on all platforms (not just Darwin), so that the build now
works on Linux, too.

pkgs/development/compilers/ghcjs/8.10/configured-ghcjs-src.nix

···
     happy
     alex
     cabal-install
-  ] ++ lib.optionals stdenv.isDarwin [
-    gcc # https://github.com/ghcjs/ghcjs/issues/663
+    gcc
   ];
   inherit ghcjsSrc;
 } ''