stdenv: switch default from gcc10 to gcc11

This will mainly affect ARM which is still using gcc10;

x86_64 (darwin and linux), RISC-V (linux), and aarch64 (darwin) are
already using gcc11 so this will remove their version override.

aarch64 (linux), vc4, and relibc will remain pinned to previous versions
due to unresolved issues.

pkgs/top-level/all-packages.nix

@@ -12500,10 +12500,8 @@ with pkgs;
   inherit (let
       num =
         if (with stdenv.targetPlatform; isVc4 || libc == "relibc") then 6
-        else if (stdenv.targetPlatform.isAarch64 && stdenv.isDarwin) then 11
-        else if (stdenv.targetPlatform.isx86_64 || stdenv.targetPlatform.isRiscV) then 11
-        else if stdenv.targetPlatform.isAarch64 then 9
-        else 10;
+        else if (stdenv.targetPlatform.isAarch64 && stdenv.isLinux) then 9
+        else 11;
       numS = toString num;
     in {
       gcc = pkgs.${"gcc${numS}"};