llvmPackages_20.libc-full: fix building (#385706)

lib/systems/default.nix

···
       # don't support dynamic linking, but don't get the `staticMarker`.
       # `pkgsStatic` sets `isStatic=true`, so `pkgsStatic.hostPlatform` always
       # has the `staticMarker`.
-      isStatic = final.isWasi || final.isRedox;
+      isStatic = final.isWasi || final.isRedox || final.isLLVMLibc;
 
       # Just a guess, based on `system`
       inherit

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

···
   overrideCC,
   wrapCCWith,
   wrapBintoolsWith,
+  buildPackages,
   buildLlvmTools, # tools, but from the previous stage, for cross
   targetLlvmLibraries, # libraries, but from the next stage, for cross
   targetLlvm,
···
           libcxx = null;
           bintools = bintoolsNoLibc';
           extraPackages = [ ];
-          extraBuildCommands =
-            lib.optionalString (lib.versions.major metadata.release_version == "13") ''
-              echo "-nostartfiles" >> $out/nix-support/cc-cflags
-            ''
-            + mkExtraBuildCommands0 cc;
+          # "-nostartfiles" used to be needed for pkgsLLVM, causes problems so don't include it.
+          extraBuildCommands = mkExtraBuildCommands0 cc;
         }
         // lib.optionalAttrs (lib.versionAtLeast metadata.release_version "14") {
-          nixSupport.cc-cflags =
-            [ "-nostartfiles" ]
-            ++ lib.optional (
-              lib.versionAtLeast metadata.release_version "15" && stdenv.targetPlatform.isWasm
-            ) "-fno-exceptions";
+          # "-nostartfiles" used to be needed for pkgsLLVM, causes problems so don't include it.
+          nixSupport.cc-cflags = lib.optional (
+            lib.versionAtLeast metadata.release_version "15" && stdenv.targetPlatform.isWasm
+          ) "-fno-exceptions";
         }
       );
 
···
           # Use clang due to "gnu::naked" not working on aarch64.
           # Issue: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77882
           stdenv = overrideCC stdenv buildLlvmTools.clangNoLibcNoRt;
+          cmake =
+            if stdenv.targetPlatform.libc == "llvm" then buildPackages.cmakeMinimal else buildPackages.cmake;
+          python3 =
+            if stdenv.targetPlatform.libc == "llvm" then
+              buildPackages.python3Minimal
+            else
+              buildPackages.python3;
         };
 
         libc = if stdenv.targetPlatform.libc == "llvm" then libraries.libc-full else libraries.libc-overlay;

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

···
     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
···
   outputs = [ "out" ] ++ (lib.optional isFullBuild "dev");
 
   postUnpack = lib.optionalString isFullBuild ''
+    chmod +w $sourceRoot/../$pname/utils/hdrgen
     patchShebangs $sourceRoot/../$pname/utils/hdrgen/main.py
     chmod +x $sourceRoot/../$pname/utils/hdrgen/main.py
   '';
···
     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")
-  ];
+  cmakeFlags =
+    [
+      (lib.cmakeBool "LLVM_LIBC_FULL_BUILD" isFullBuild)
+      (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.
+      # This includes including headers which requires a libc.
+      # Skip these checks because a libc cannot be used when one doesn't exist.
+      (lib.cmakeBool "CMAKE_C_COMPILER_WORKS" true)
+      (lib.cmakeBool "CMAKE_CXX_COMPILER_WORKS" true)
+    ];
 
   # For the update script:
   passthru = {