xtool: utilize llvm-config to find LLVM executables

xtool/env/llvm/llvm_config_byollvm.go

@@ -0,0 +1,26 @@
+//go:build byollvm
+
+/*
+ * Copyright (c) 2024 The GoPlus Authors (goplus.org). All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// The word "byollvm" stands for "Bring Your Own LLVM" and is a build tag used
+// to indicate that the package is being built with a custom LLVM installation.
+
+package llvm
+
+// ldLLVMConfigBin is the path to the llvm-config binary. It shoud be set via
+// -ldflags when building the package.
+var ldLLVMConfigBin string