stdenv: use gcc9 for aarch64

Several aarch64 builds fail with is referenced by DSO followed by
bin/ld: final link failed: bad value. This seems related to libgcc_s.so.

https://github.com/NixOS/nixpkgs/issues/108111

pkgs/top-level/all-packages.nix

@@ -9180,7 +9180,8 @@ in
     then ../development/compilers/gcc/6
     else ../development/compilers/gcc/10);
   gcc = if (with stdenv.targetPlatform; isVc4 || libc == "relibc")
-    then gcc6 else gcc10;
+    then gcc6 else
+      if stdenv.targetPlatform.isAarch64 then gcc9 else gcc10;
   gcc-unwrapped = gcc.cc;
 
   gccStdenv = if stdenv.cc.isGNU then stdenv else stdenv.override {