Distinguish pkgsStatic from pkgsMusl via stdenv.targetPlatform

This change allows derivations to distinguish dynamic musl build and
static musl build in cases where upstream build system can't detect it
by itself.

lib/systems/default.nix

@@ -76,6 +76,7 @@ rec {
          # uname -r
          release = null;
       };
+      isStatic = final.isWasm || final.isRedox;
 
       kernelArch =
         if final.isAarch32 then "arm"

pkgs/top-level/stage.nix

@@ -215,6 +215,7 @@ let
       crossOverlays = [ (import ./static.nix) ];
     } // lib.optionalAttrs stdenv.hostPlatform.isLinux {
       crossSystem = {
+        isStatic = true;
         parsed = stdenv.hostPlatform.parsed // {
           abi = {
             gnu = lib.systems.parse.abis.musl;