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/nintendoswitch.json

@@ -0,0 +1,35 @@
+{
+  "llvm-target": "aarch64",
+  "cpu": "cortex-a57",
+  "features": "+aes,+crc,+fp-armv8,+neon,+perfmon,+sha2,+v8a,-fmv",
+  "build-tags": ["nintendoswitch", "arm64"],
+  "scheduler": "tasks",
+  "goos": "linux",
+  "goarch": "arm64",
+  "linker": "ld.lld",
+  "rtlib": "compiler-rt",
+  "libc": "picolibc",
+  "gc": "conservative",
+  "relocation-model": "pic",
+  "default-stack-size": 2048,
+  "cflags": [
+    "-target", "aarch64-unknown-none",
+    "-fPIE",
+    "-Werror",
+    "-fshort-enums",
+    "-fomit-frame-pointer",
+    "-fno-exceptions", "-fno-unwind-tables", "-fno-asynchronous-unwind-tables",
+    "-ffunction-sections", "-fdata-sections"
+  ],
+  "ldflags": [
+    "-pie",
+    "-z", "notext"
+  ],
+  "linkerscript": "targets/nintendoswitch.ld",
+  "extra-files": [
+    "targets/nintendoswitch.s",
+    "src/internal/task/task_stack_arm64.S",
+    "src/runtime/asm_arm64.S",
+    "src/runtime/runtime_nintendoswitch.S"
+  ]
+}