feat: implement target configuration system for issue #1176

Add comprehensive target configuration parsing and inheritance system:

- Create internal/targets package with config structures
- Support JSON configuration loading with inheritance resolution
- Implement multi-level inheritance (e.g., rp2040 → cortex-m0plus → cortex-m)
- Add 206 target configurations from TinyGo for embedded platforms
- Support core fields: name, llvm-target, cpu, features, build-tags, goos, goarch, cflags, ldflags
- Provide high-level resolver interface for target lookup
- Include comprehensive unit tests with 100% target parsing coverage

This foundation enables future -target parameter support for cross-compilation
to diverse embedded platforms beyond current GOOS/GOARCH limitations.

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

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

targets/riscv.json

@@ -0,0 +1,25 @@
+{
+	"goos": "linux",
+	"goarch": "arm",
+	"build-tags": ["tinygo.riscv", "baremetal", "linux", "arm"],
+	"gc": "conservative",
+	"linker": "ld.lld",
+	"rtlib": "compiler-rt",
+	"libc": "picolibc",
+	"cflags": [
+		"-Werror",
+		"-mno-relax",
+		"-fno-exceptions", "-fno-unwind-tables", "-fno-asynchronous-unwind-tables",
+		"-ffunction-sections", "-fdata-sections"
+	],
+	"ldflags": [
+		"--gc-sections"
+	],
+	"extra-files": [
+		"src/device/riscv/start.S",
+		"src/internal/task/task_stack_tinygoriscv.S",
+		"src/runtime/asm_riscv.S",
+		"src/device/riscv/handleinterrupt.S"
+	],
+	"gdb": ["riscv64-unknown-elf-gdb"]
+}