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/avrtiny.S

@@ -0,0 +1,43 @@
+#define __tmp_reg__ r16
+#define __zero_reg__ r17
+
+; Startup code
+.section .text.__vector_RESET
+.global  __vector_RESET
+__vector_RESET:
+    clr  __zero_reg__ ; this register is expected to be 0 by the C calling convention
+
+    ; Keep the stack pointer at the default location, which is RAMEND.
+
+; Initialize .data section.
+.section .text.__do_copy_data,"ax",@progbits
+.global __do_copy_data
+__do_copy_data:
+    ldi  xl, lo8(__data_start)
+    ldi  xh, hi8(__data_start)
+    ldi  yl, lo8(__data_end)
+    ldi  yh, hi8(__data_end)
+    ldi  zl, lo8(__data_load_start)
+    ldi  zh, hi8(__data_load_start)
+1: ; loop
+    cp   xl, yl         ; if x == y
+    cpc  xh, yh
+    breq 2f             ; goto end
+    ld   r16, Z+        ; r0 = *(z++)
+    st   X+, r16        ; *(x++) = r0
+    rjmp 1b             ; goto loop
+2: ; end
+
+; Initialize .bss section.
+.section .text.__do_clear_bss,"ax",@progbits
+.global __do_clear_bss
+__do_clear_bss:
+    ldi  xl, lo8(__bss_start)
+    ldi  xh, hi8(__bss_start)
+    ldi  yl, lo8(__bss_end)
+1: ; loop
+    cp   xl, yl           ; if x == y
+    breq 2f               ; goto end
+    st   X+, __zero_reg__ ; *(x++) = 0
+    rjmp 1b               ; goto loop
+2: ; end