docs: add LICENSE file for targets directory

Add license attribution for target configuration files derived from TinyGo project:

- Document source from https://github.com/tinygo-org/tinygo/tree/release/targets
- Include complete BSD 3-Clause license from TinyGo project
- Clarify licensing terms for target configuration files
- Ensure proper attribution to TinyGo Authors and Go Authors
- Maintain compliance with original license requirements

This ensures proper license compliance when using TinyGo's target configurations
in the llgo project.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

targets/LICENSE

@@ -0,0 +1,61 @@
+Target Configuration Files License
+=====================================
+
+The target configuration files in this directory are derived from the TinyGo project
+(https://github.com/tinygo-org/tinygo/tree/release/targets) and are subject to the
+TinyGo project's license terms.
+
+Original source: https://github.com/tinygo-org/tinygo/tree/release/targets
+License: BSD 3-Clause License (same as TinyGo project)
+
+--------------------------------------------------------------------------------
+
+TinyGo License (BSD 3-Clause)
+==============================
+
+Copyright (c) 2018-2025 The TinyGo Authors. All rights reserved.
+
+TinyGo includes portions of the Go standard library.
+Copyright (c) 2009-2024 The Go Authors. All rights reserved.
+
+TinyGo includes portions of LLVM, which is under the Apache License v2.0 with
+LLVM Exceptions. See https://llvm.org/LICENSE.txt for license information.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+   * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+   * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+   * Neither the name of the copyright holder nor the names of its
+contributors may be used to endorse or promote products derived from
+this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+--------------------------------------------------------------------------------
+
+Usage in llgo project
+======================
+
+These target configuration files have been adapted for use in the llgo project
+to provide cross-compilation support for embedded platforms. The files maintain
+their original structure and content while being integrated into llgo's build
+system.
+
+Any modifications or adaptations to these files for llgo-specific use are
+released under the same BSD 3-Clause license terms as the original TinyGo project.