boringssl: support cross compile

set GOARCH to allow codegen programs to run on the buildPlatform

pkgs/development/libraries/boringssl/default.nix

@@ -26,6 +26,8 @@ buildGoModule {
   # (if we use postConfigure then cmake will loop runHook postConfigure)
   preBuild = ''
     cmakeConfigurePhase
+  '' + lib.optionalString (stdenv.buildPlatform != stdenv.hostPlatform) ''
+    export GOARCH=$(go env GOHOSTARCH)
   '';
 
   buildPhase = ''