gcc: for cross compilers, don't build libgcc twice

Cross-compiled binaries currently end up with two different libgcc
outpaths in their closure.  This is harmless, but confusing.

The two libgccs are:

- One of them is the "first" targetPlatform libgcc, which is built
  by the "first" cross-compiler.  This "first libgcc" and "first
  compiler" are used to build the targetPlatform glibc.

- Once glibc is built, we *rebuild* the cross-compiler, since gcc
  can't enable most of its features unless you give it an
  already-compiled targetPlatform glibc.  When this "second"
  compiler is built, it also builds an extra copy of libgcc.

This commit discards the second, extra libgcc, and instead puts a
reference to the first (correct) libgcc into the "second compiler"
`.passthru.libgcc`, so that anybody expecting `stdenv.cc.cc.libgcc`
to exist will still find it there.

Closes #249680

pkgs/development/compilers/gcc/common/libgcc.nix

@@ -8,6 +8,7 @@
 , targetPlatform
 , hostPlatform
 , withoutTargetLibc
+, libcCross
 }:
 
 assert !stdenv.targetPlatform.hasSharedLibraries -> !enableShared;
@@ -31,7 +32,7 @@ drv: lib.pipe drv
 # nixpkgs did not add the "libgcc" output until gcc11.  In theory
 # the following condition can be changed to `true`, but that has not
 # been tested.
-lib.optional (lib.versionAtLeast version "11.0")
+lib.optionals (lib.versionAtLeast version "11.0")
 
 (let
   targetPlatformSlash =
@@ -39,11 +40,20 @@ lib.optional (lib.versionAtLeast version "11.0")
     then ""
     else "${targetPlatform.config}/";
 
+  # If we are building a cross-compiler and the target libc provided
+  # to us at build time has a libgcc, use that instead of building a
+  # new one.  This avoids having two separate (but identical) libgcc
+  # outpaths in the closure of most packages, which can be confusing.
+  useLibgccFromTargetLibc =
+    libcCross != null &&
+    libcCross?passthru.libgcc;
+
   enableLibGccOutput =
     (!stdenv.targetPlatform.isWindows || (with stdenv; targetPlatform == hostPlatform)) &&
     !langJit &&
     !stdenv.hostPlatform.isDarwin &&
-    enableShared
+    enableShared &&
+    !useLibgccFromTargetLibc
   ;
 
   # For some reason libgcc_s.so has major-version "2" on m68k but
@@ -54,6 +64,14 @@ lib.optional (lib.versionAtLeast version "11.0")
     else "1";
 
 in
+[
+
+(pkg: pkg.overrideAttrs (previousAttrs: lib.optionalAttrs useLibgccFromTargetLibc {
+  passthru = (previousAttrs.passthru or {}) // {
+    inherit (libcCross) libgcc;
+  };
+}))
+
 (pkg: pkg.overrideAttrs (previousAttrs: lib.optionalAttrs ((!langC) || langJit || enableLibGccOutput) {
   outputs = previousAttrs.outputs ++ lib.optionals enableLibGccOutput [ "libgcc" ];
   # This is a separate phase because gcc assembles its phase scripts
@@ -143,5 +161,4 @@ in
     + ''
       patchelf --set-rpath "" $libgcc/lib/libgcc_s.so.${libgcc_s-version-major}
     '');
-}))))
-
+}))]))

pkgs/development/compilers/gcc/default.nix

@@ -450,7 +450,7 @@ ${""}          done
 }
 ))
 ([
-  (callPackage ./common/libgcc.nix   { inherit version langC langCC langJit targetPlatform hostPlatform withoutTargetLibc enableShared; })
+  (callPackage ./common/libgcc.nix   { inherit version langC langCC langJit targetPlatform hostPlatform withoutTargetLibc enableShared libcCross; })
 ] ++ optionals atLeast11 [
   (callPackage ./common/checksum.nix { inherit langC langCC; })
 ])