llvmPackages_20.libc: enable scudo & link Scrt1

pkgs/development/compilers/llvm/common/libc/default.nix

@@ -23,6 +23,7 @@ let
     cp -r ${monorepoSrc}/cmake "$out"
     cp -r ${monorepoSrc}/runtimes "$out"
     cp -r ${monorepoSrc}/llvm "$out"
+    cp -r ${monorepoSrc}/compiler-rt "$out"
     cp -r ${monorepoSrc}/${pname} "$out"
   '');
 in
@@ -60,18 +61,28 @@ stdenv.mkDerivation (finalAttrs: {
     cd ../runtimes
   '';
 
-  postInstall = lib.optionalString (!isFullBuild) ''
-    substituteAll ${./libc-shim.tpl} $out/lib/libc.so
-  '';
+  postInstall =
+    lib.optionalString (!isFullBuild) ''
+      substituteAll ${./libc-shim.tpl} $out/lib/libc.so
+    ''
+    # LLVM libc doesn't recognize static vs dynamic yet.
+    # Treat LLVM libc as a static libc, requires this symlink until upstream fixes it.
+    + lib.optionalString (isFullBuild && stdenv.hostPlatform.isLinux) ''
+      ln $out/lib/crt1.o $out/lib/Scrt1.o
+    '';
 
   libc = if (!isFullBuild) then stdenv.cc.libc else null;
 
   cmakeFlags =
     [
       (lib.cmakeBool "LLVM_LIBC_FULL_BUILD" isFullBuild)
-      (lib.cmakeFeature "LLVM_ENABLE_RUNTIMES" "libc")
+      (lib.cmakeFeature "LLVM_ENABLE_RUNTIMES" "libc;compiler-rt")
       # Tests requires the host to have a libc.
       (lib.cmakeBool "LLVM_INCLUDE_TESTS" (stdenv.cc.libc != null))
+      (lib.cmakeBool "LLVM_LIBC_INCLUDE_SCUDO" true)
+      (lib.cmakeBool "COMPILER_RT_BUILD_SCUDO_STANDALONE_WITH_LLVM_LIBC" true)
+      (lib.cmakeBool "COMPILER_RT_BUILD_GWP_ASAN" false)
+      (lib.cmakeBool "COMPILER_RT_SCUDO_STANDALONE_BUILD_SHARED" false)
     ]
     ++ lib.optional (isFullBuild && stdenv.cc.libc == null) [
       # CMake runs a check to see if the compiler works.