gcc: do not allow version skew when cross-building gcc

When `gcc` is cross-built (`build` != `target` && `host` == `target`)
`gcc` assumes that it has a compatible cross-compiler in the environment
that can build target libraries. Version of a cross-compiler has to
match the compiler being cross-built as libraries frequently use fresh
compiler features, like `-std=c++26` or target-specific types like
`_Bfloat16`.

Version mismatch causes build failures like:

    #351905

Similar problems (but on a smaller scale) happen when a `gcc`
cross-compiler is built (`build` == `host` && `host` != `target`) built
by a mismatching version of a native compiler (`build` == `host` &&
`host` == `target`). That was worked around by forcing `gcc9Stdenv` for
older compiler versions.

Let's fix both problems by requiring the same compiler version for
cross-case.

Closes: #351905

pkgs/development/compilers/gcc/all.nix

@@ -1,7 +1,6 @@
 { lib
 , stdenv
-, gccStdenv
-, gcc9Stdenv
+, pkgs
 , callPackage
 , isl_0_17, isl_0_20
 , libcCross
@@ -15,6 +14,7 @@ let
   versions = import ./versions.nix;
   gccForMajorMinorVersion = majorMinorVersion:
     let
+      majorVersion = lib.versions.major majorMinorVersion;
       atLeast = lib.versionAtLeast majorMinorVersion;
       attrName = "gcc${lib.replaceStrings ["."] [""] majorMinorVersion}";
       pkg = lowPrio (wrapCC (callPackage ./default.nix ({
@@ -27,9 +27,25 @@ let
         isl = if       stdenv.hostPlatform.isDarwin then null
               else if    atLeast "9"   then isl_0_20
               else    /* atLeast "7" */     isl_0_17;
-      } // lib.optionalAttrs (!(atLeast "9")) {
-        # gcc 10 is too strict to cross compile gcc <= 8
-        stdenv = if (stdenv.targetPlatform != stdenv.buildPlatform) && stdenv.cc.isGNU then gcc9Stdenv else stdenv;
+        # do not allow version skew when cross-building gcc
+        #
+        # When `gcc` is cross-built (`build` != `target` && `host` == `target`)
+        # `gcc` assumes that it has a compatible cross-compiler in the environment
+        # that can build target libraries. Version of a cross-compiler has to
+        # match the compiler being cross-built as libraries frequently use fresh
+        # compiler features, like `-std=c++26` or target-specific types like
+        # `_Bfloat16`.
+        # Version mismatch causes build failures like:
+        #     https://github.com/NixOS/nixpkgs/issues/351905
+        #
+        # Similar problems (but on a smaller scale) happen when a `gcc`
+        # cross-compiler is built (`build` == `host` && `host` != `target`) built
+        # by a mismatching version of a native compiler (`build` == `host` &&
+        # `host` == `target`).
+        #
+        # Let's fix both problems by requiring the same compiler version for
+        # cross-case.
+        stdenv = if (stdenv.targetPlatform != stdenv.buildPlatform) && stdenv.cc.isGNU then pkgs."gcc${majorVersion}Stdenv" else stdenv;
       })));
     in
       lib.nameValuePair attrName pkg;