internal/rosa: patch header search paths

This removes the need for reference CFLAGS in the standard toolchain.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/llvm.go

@@ -310,13 +310,19 @@ ln -s \
 		},
 	})
 
+	var clangExtraEnv []string
+	if t == toolchainStage3 {
+		clangExtraEnv = []string{
+			"CFLAGS=" + earlyCFLAGS,
+			"CXXFLAGS=" + earlyCXXFLAGS(),
+		}
+	}
+
 	clang = t.newLLVM("clang", &llvmAttr{
 		flags: llvmProjectClang | llvmProjectLld,
-		env: []string{
-			"CFLAGS=" + cflags,
-			"CXXFLAGS=" + cxxflags(),
+		env: slices.Concat(clangExtraEnv, []string{
 			ldflags(false),
-		},
+		}),
 		cmake: slices.Concat([][2]string{
 			{"LLVM_TARGETS_TO_BUILD", target},
 			{"CMAKE_CROSSCOMPILING", "OFF"},
@@ -329,6 +335,9 @@ ln -s \
 			t.NewGit(),
 		},
 		script: `
+ln -s clang /work/system/bin/cc
+ln -s clang++ /work/system/bin/c++
+
 ninja check-all
 `,
 
@@ -353,6 +362,33 @@ index 50fdce630255..4b4465a75617 100644
  /// Verify timestamps that gets embedded in the module
  #include <c-header.h>
 
+`},
+
+			{"path-system-include", `diff --git a/clang/lib/Driver/ToolChains/Linux.cpp b/clang/lib/Driver/ToolChains/Linux.cpp
+index cdbf21fb9026..dd052858700d 100644
+--- a/clang/lib/Driver/ToolChains/Linux.cpp
++++ b/clang/lib/Driver/ToolChains/Linux.cpp
+@@ -773,6 +773,12 @@ void Linux::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
+     addExternCSystemInclude(
+         DriverArgs, CC1Args,
+         concat(SysRoot, "/usr/include", MultiarchIncludeDir));
++  if (!MultiarchIncludeDir.empty() &&
++      D.getVFS().exists(concat(SysRoot, "/system/include", MultiarchIncludeDir)))
++    addExternCSystemInclude(
++        DriverArgs, CC1Args,
++        concat(SysRoot, "/system/include", MultiarchIncludeDir));
++
+
+   if (getTriple().getOS() == llvm::Triple::RTEMS)
+     return;
+@@ -783,6 +789,7 @@ void Linux::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
+   addExternCSystemInclude(DriverArgs, CC1Args, concat(SysRoot, "/include"));
+
+   addExternCSystemInclude(DriverArgs, CC1Args, concat(SysRoot, "/usr/include"));
++  addExternCSystemInclude(DriverArgs, CC1Args, concat(SysRoot, "/system/include"));
+
+   if (!DriverArgs.hasArg(options::OPT_nobuiltininc) && getTriple().isMusl())
+     addSystemInclude(DriverArgs, CC1Args, ResourceDirInclude);
 `},
 		},
 	})