tests.cross.sanity: Add GHC to catch regressions

This is to ensure that Haskell users on platforms that lack official
bindists still have a convenient means of getting GHC running natively.

In my admittedly somewhat limited testing on RISC-V, GHC 8.10.7 is able
to bootstrap native builds for 9.2.8 and 9.4.5. GHC 9.2.8 and 9.4.5 are
unable to bootstrap themselves and 9.6.2 when cross-compiled.

If you're looking at this commit to see whether you can safely upgrade
the compiler used here to remove 8.10, please try cross-compiling 9.0 or
later and then booting a native GHC with it.

pkgs/test/cross/default.nix

@@ -146,6 +146,8 @@ let
     pkgs.pkgsCross.mips64el-linux-gnuabi64.stdenv
     pkgs.pkgsCross.mips64el-linux-gnuabin32.stdenv
     pkgs.pkgsCross.mingwW64.stdenv
+    # Unlike later versions, cross-compiled 8.10.7 can reliably bootstrap a native build
+    pkgs.pkgsCross.riscv64.haskell.compiler.integer-simple.ghc8107
 
   ] ++ lib.optionals (with pkgs.stdenv.buildPlatform; isx86_64 && isLinux) [
     # Musl-to-glibc cross on the same architecture tends to turn up