llvm_12: backport patch for musl 1.2.4

pkgs/development/compilers/llvm/12/llvm/default.nix

···
       sha256 = "sha256:12s8vr6ibri8b48h2z38f3afhwam10arfiqfy4yg37bmc054p5hi";
       stripLen = 1;
     })
+
+    # Fix musl build.
+    (fetchpatch {
+      url = "https://github.com/llvm/llvm-project/commit/5cd554303ead0f8891eee3cd6d25cb07f5a7bf67.patch";
+      relative = "llvm";
+      hash = "sha256-XPbvNJ45SzjMGlNUgt/IgEvM2dHQpDOe6woUJY+nUYA=";
+    })
   ] ++ lib.optional enablePolly ./gnu-install-dirs-polly.patch;
 
   postPatch = optionalString stdenv.isDarwin ''