feat(libbboeos): link clang-compiled exports into the shared blob

[bboe]

Summary

First concrete step toward Phase 3 of the shared-libbboeos design (spec). Wire `user/libbboeos/libbboeos.a` (clang-compiled libc bodies) into the same `build/libbboeos` blob as the asm vDSO helpers, gated by `--gc-sections` + `-ffunction-sections` so unreferenced functions are dropped.

Concretely:

• `user/libbboeos/Makefile` gains `-ffunction-sections -fdata-sections`.
• `libbboeos.ld` absorbs `(.text .text.)` / `(.rodata .rodata.)` into the existing output sections; new `EXTERN(strcmp)` forces archive resolution. Pointer table grows by one `LONG(strcmp)` entry at offset 0x34.
• `make_os.sh` links the .a alongside the nasm-built vdso.o:
  ```
  ld -m elf_i386 -T libbboeos.ld --gc-sections -o libbboeos.elf libbboeos.o libbboeos.a
  ```
• `constants.asm` exposes `FUNCTION_STRCMP_PTR`.
• `VDSO_SIGRETURN_OFFSET` moves `0x460` → `0xFE0` so the helper region can grow past 1 KB. Sigreturn now sits near the end of page 0, past the pointer table at `0x800..0x83C`. No ABI break — kernel `signal_dispatch_user` references the constant symbolically.

`build/libbboeos` grows from 2100 → 4068 bytes (still single-page; strcmp body is 0x3D bytes, the rest of the growth is the sigreturn relocation extending the file out to 0xFE4).

What's NOT in this PR

The cc.py-side wiring — the extern-call fallback that emits `call [FUNCTION__PTR]` for unknown names — lands in a follow-up. This commit is purely the build-pipeline plumbing that makes `strcmp` reachable in the blob.

Test plan

• `./make_os.sh` clean
• `build/libbboeos.map` shows `strcmp` at `0x10410`, pointer at `0x10834` (= `FUNCTION_STRCMP_PTR`)
• `tests/test_asm.py` — 42/42 pass
• `tests/test_programs.py` — 89/89 pass (signal-handling exercise via shell intact post-sigreturn relocation)
• `tests/test_bboefs.py` — 6/6 pass
• `tests/test_pipeline_*.py` — 7/7 pass
• `pytest tests/unit/` — 455/455 pass

🤖 Generated with Claude Code