gcc6: don’t link libstdc++ to CoreFoundation

Using flat namespaces causes libstdc++ to link CoreFoundation, but
that fails after #265102. Since CoreFoundation is not actually needed,
disable flat namespaces to avoid linking it unnecessarily.

Disabling flat namespaces matches the behavior of newer versions of
libstdc++ (GCC 7+) when building for newer Darwin hosts (10.5+).

pkgs/development/compilers/gcc/patches/6/libstdc++-disable-flat_namespace.patch

@@ -0,0 +1,26 @@
+Backported from GCC 7.
+
+diff --git a/libstdc++-v3/configure.host b/libstdc++-v3/configure.host
+index 304a7f5aff6..d1a189d93d0 100644
+--- a/libstdc++-v3/configure.host
++++ b/libstdc++-v3/configure.host
+@@ -234,7 +234,7 @@ case "${host_os}" in
+     os_include_dir="os/newlib"
+     OPT_LDFLAGS="${OPT_LDFLAGS} \$(lt_host_flags)"
+     ;;
+-  darwin | darwin[1-7] | darwin[1-7].*)
++  darwin[1-7] | darwin[1-7].*)
+     # On Darwin, performance is improved if libstdc++ is single-module.
+     # Up to at least 10.3.7, -flat_namespace is required for proper
+     # treatment of coalesced symbols.
+@@ -252,6 +252,10 @@ case "${host_os}" in
+     esac
+     os_include_dir="os/bsd/darwin"
+     ;;
++  darwin*)
++    # Post Darwin8, defaults should be sufficient.
++    os_include_dir="os/bsd/darwin"
++    ;;
+   *djgpp*)      # leading * picks up "msdosdjgpp"
+     os_include_dir="os/djgpp"
+     error_constants_dir="os/djgpp"

pkgs/development/compilers/gcc/patches/default.nix

@@ -259,6 +259,9 @@ in
 # This patch can be dropped should darwin.cctools-llvm ever implement support.
 ++ optional (!atLeast7 && hostPlatform.isDarwin && lib.versionAtLeast (lib.getVersion stdenv.cc) "12") ./4.9/darwin-clang-as.patch
 
+# Building libstdc++ with flat namespaces results in trying to link CoreFoundation, which
+# defaults to the impure, system location and causes the build to fail.
+++ optional (is6 && hostPlatform.isDarwin) ./6/libstdc++-disable-flat_namespace.patch
 
 ## gcc 4.9 and older ##############################################################################