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-qemu.ld

@@ -0,0 +1,16 @@
+
+/* Memory map:
+ * https://github.com/qemu/qemu/blob/master/hw/riscv/virt.c
+ * Looks like we can use any address starting from 0x80000000 (so 2GB of space).
+ * However, using a large space slows down tests.
+ */
+MEMORY
+{
+    RAM (rwx) : ORIGIN = 0x80000000, LENGTH = 100M
+}
+
+REGION_ALIAS("FLASH_TEXT", RAM)
+
+_stack_size = 2K;
+
+INCLUDE "targets/riscv.ld"