emscripten: patch bugs involving EM_CACHE permissions

As a workaround for https://github.com/NixOS/nixpkgs/issues/282509,
packages built with `emscriptenStdenv` need to set the `EM_CACHE`
environment variable to a folder in the build directory, so that
emscripten uses a copy of the sysroot cache instead of attempting to
write to the sysroot in the Nix store. However, due to the bug
https://github.com/emscripten-core/emscripten/issues/24404, the copied
cache has the same permission bits as the (readonly) one in the Nix
store, resulting in "Permission Denied" errors.

This bug has since been fixed in upstream emscripten, so we can backport
the relevant patches.

pkgs/development/compilers/emscripten/default.nix

@@ -2,6 +2,7 @@
   lib,
   stdenv,
   fetchFromGitHub,
+  fetchpatch,
   python3,
   nodejs,
   closurecompiler,
@@ -60,6 +61,17 @@ stdenv.mkDerivation rec {
     (replaceVars ./0001-emulate-clang-sysroot-include-logic.patch {
       resourceDir = "${llvmEnv}/lib/clang/${lib.versions.major llvmPackages.llvm.version}/";
     })
+    # The following patches work around a bug where EM_CACHE is not copied with
+    # the correct permissions; the bug will be fixed in the next release (probably 4.0.10).
+    # See also: https://github.com/emscripten-core/emscripten/issues/24404
+    (fetchpatch {
+      url = "https://github.com/emscripten-core/emscripten/commit/99c6e41154f701e423074e33a4fdaf5eea49d073.patch";
+      hash = "sha256-/wkhz08NhbgxsrXd7YFfdCGX6LrS2Ncct8dcwxBMsjY=";
+    })
+    (fetchpatch {
+      url = "https://github.com/emscripten-core/emscripten/commit/f4d358d740a238b67a1d6935e71638519d25afa0.patch";
+      hash = "sha256-hib5ZAN/R2dH+rTv3nYF37+xKZmeboKxnS+5mkht2lM=";
+    })
   ];
 
   buildPhase = ''