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/riscv64.json

@@ -0,0 +1,13 @@
+{
+	"inherits": ["riscv"],
+	"llvm-target": "riscv64-unknown-none",
+	"cpu": "generic-rv64",
+	"target-abi": "lp64",
+	"build-tags": ["tinygo.riscv64"],
+	"cflags": [
+		"-march=rv64gc"
+	],
+	"ldflags": [
+		"-melf64lriscv"
+	]
+}