feat(cc): extern-call fallback for libbboeos exports

[bboe]

Summary

Phase 3 step 2 of the shared-libbboeos design (spec). cc.py's `Call` AST visitor previously raised `unknown function: ` whenever a called identifier wasn't a builtin / user function / function pointer / inline body. When the called name matches a defined `FUNCTION__PTR` constant in `kernel/include/constants.asm` under `--target user`, emit a cdecl indirect call through the libbboeos pointer table:

```
push ... push
call [FUNCTION__PTR]
add esp, N * 4
```

The pointer table lives at user-virt `0x10800` in the libbboeos blob (populated by `user/libbboeos/libbboeos.ld`'s `LONG()` entries); each `FUNCTION__PTR` is the absolute address of its slot. Same clobber accounting as a user-function call (full register-pool caller-saves charged) so the auto-pin allocator schedules around the call site correctly.

Self-host assembler

The self-hosted assembler in `user/programs/asm.c` gains the matching `call [absolute]` form — `FF 15 abs32` under bits=32, `FF 16 abs16` under bits=16 — so `tests/test_asm.py`'s NASM-vs-self-host byte-equivalence harness keeps clean across the seven programs that now use the new emission.

Cleanups

Per-program `strcmp` reimplementations in:

• `user/programs/fd_helpers.c`
• `user/programs/ls.c`
• `user/programs/pipe_consumer.c`
• `user/programs/pipe_producer.c`
• `user/programs/pipe_spam.c`
• `user/programs/shell.c`
• `user/programs/sort.c` (both the forward decl + body)

…are deleted; those calls now resolve to the libbboeos shared `strcmp` via `FUNCTION_STRCMP_PTR`.

Kernel-mode compilation keeps the strict unknown-function error (the kernel must not reach into the user shared blob).

Test plan

• `./make_os.sh` clean
• `tests/test_asm.py` — 42/42 pass (round-trip including `call [FUNCTION_STRCMP_PTR]`)
• `tests/test_programs.py` — 89/89
• `tests/test_bboefs.py` — 6/6
• `tests/test_pipeline_*.py` — 7/7 (the shell + pipe programs are the heaviest strcmp users)
• `tests/test_cc_compatibility.py` — 57/57 (clang's strcmp prototype from `<string.h>` matches the call sites)
• `pytest tests/unit/` — 456/456

🤖 Generated with Claude Code