builder: also check lib64 for clang include path.

On 64-bit Fedora, `lib64` is where the clang headers are, not `lib`. For
multiarch systems, both will exist, but it's likely you want 64-bit, so
check that first.

builder/env.go

@@ -40,14 +40,20 @@ func getClangHeaderPath(TINYGOROOT string) string {
 			// Example executable:
 			//     /usr/lib/llvm-9/bin/clang
 			// Example include path:
-			//     /usr/lib/llvm-9/lib/clang/9.0.1/include/
+			//     /usr/lib/llvm-9/lib64/clang/9.0.1/include/
 			llvmRoot := filepath.Dir(filepath.Dir(binpath))
-			clangVersionRoot := filepath.Join(llvmRoot, "lib", "clang")
+			clangVersionRoot := filepath.Join(llvmRoot, "lib64", "clang")
 			dirs, err := ioutil.ReadDir(clangVersionRoot)
+			if err != nil {
+				// Example include path:
+				//     /usr/lib/llvm-9/lib/clang/9.0.1/include/
+				clangVersionRoot = filepath.Join(llvmRoot, "lib", "clang")
+				dirs, err = ioutil.ReadDir(clangVersionRoot)
 				if err != nil {
 					// Unexpected.
 					continue
 				}
+			}
 			dirnames := make([]string, len(dirs))
 			for i, d := range dirs {
 				dirnames[i] = d.Name()