pkgs/test/stdenv/default.nix: add gcc-stageCompare

This commit adds a derivation `gcc-stageCompare` to
`pkgs/test/stdenv/default.nix`.

It is important to always build this derivation whenever building
`stdenv`!  Because we are using a Nix-driven bootstrap instead of
gcc's built-in `--enable-bootstrap`, the `gcc` derivation no longer
performs the post-self-compilation sanity check.  You must build
this derivation in order to perform that sanity check.

The major benefit of this new approach is that the sanity check
(which involves a third compilation of gcc) can be performed
*concurrently* with all packages that depend on `stdenv`, rather
than serially.  Since `stdenv` has very little derivation-level
parallelism it cannot take advantage of more than one or perhaps two
builders.  If you have three or more builders this commit will
reduce the time-to-rebuild-stdenv by around 20% (one of three gcc
rebuilds is removed from the critical path, and stdenv's build time
is dominated by roughly 3*gcc + 1*binutils + 1*bison-test-suite).

Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

nixos/release-small.nix

@@ -81,7 +81,8 @@ in rec {
       stdenv
       subversion
       tarball
-      vim;
+      vim
+      tests-stdenv-gcc-stageCompare;
   };
 
   tested = let
@@ -131,6 +132,7 @@ in rec {
         "nixos.tests.proxy"
         "nixos.tests.simple"
         "nixpkgs.jdk"
+        "nixpkgs.tests-stdenv-gcc-stageCompare"
       ])
     ];
   };