gcc: set up sysroot for the Darwin SDK

The GCC bootstrap builds each stage with an unwrapped GCC, which needs
to correctly handle looking for libraries and headers in sysroot.
Setting the native header folder to `/usr/include` will actually cause
GCC to search `<sysroot>/usr/include`, which is the SDKROOT in nixpkgs.

pkgs/development/compilers/gcc/common/configure-flags.nix

@@ -5,7 +5,7 @@
 , threadsCross
 , version
 
-, binutils, gmp, mpfr, libmpc, isl
+, apple-sdk, binutils, gmp, mpfr, libmpc, isl
 
 , enableLTO
 , enableMultilib
@@ -112,7 +112,11 @@ let
     ]
     ++ lib.optionals (!withoutTargetLibc) [
       (if libcCross == null
-       then "--with-native-system-header-dir=${lib.getDev stdenv.cc.libc}/include"
+       then (
+        # GCC will search for the headers relative to SDKROOT on Darwin, so it will find them in the store.
+        if targetPlatform.isDarwin then "--with-native-system-header-dir=/usr/include"
+        else "--with-native-system-header-dir=${lib.getDev stdenv.cc.libc}/include"
+       )
        else "--with-native-system-header-dir=${lib.getDev libcCross}${libcCross.incdir or "/include"}")
       # gcc builds for cross-compilers (build != host) or cross-built
       # gcc (host != target) always apply the offset prefix to disentangle
@@ -132,7 +136,8 @@ let
       #
       # We pick "/" path to effectively avoid sysroot offset and make it work
       # as a native case.
-      "--with-build-sysroot=/"
+      # Darwin requires using the SDK as the sysroot for `SDKROOT` to work correctly.
+      "--with-build-sysroot=${if targetPlatform.isDarwin then apple-sdk.sdkroot else "/"}"
       # Same with the stdlibc++ headers embedded in the gcc output
       "--with-gxx-include-dir=${placeholder "out"}/include/c++/${version}/"
     ]

pkgs/development/compilers/gcc/default.nix

@@ -34,6 +34,7 @@
 , nukeReferences
 , callPackage
 , majorMinorVersion
+, apple-sdk
 , cctools
 , darwin
 }:
@@ -105,6 +106,7 @@ let
       ;
       # inherit generated with 'nix eval --json --impure --expr "with import ./. {}; lib.attrNames (lib.functionArgs gcc${majorVersion}.cc.override)" | jq '.[]' --raw-output'
       inherit
+        apple-sdk
         binutils
         buildPackages
         cargo