codegen/link(macho): PIE Mach-O executable writer + rebase stream (Apple Silicon arm64 exec)

[octalide]

Part of #1718. Blocks the aarch64 leg of #1680.

What

macOS arm64 (Apple Silicon) refuses to exec a non-PIE main executable — proven on
macos-14 in #1718 (the linker even forces PIE: -no_pie ignored for arm64). mach
emits a non-PIE fixed-absolute image (__TEXT at 0xfffff000, empty rebase stream),
so its arm64 darwin binaries are Killed: 9 at exec. This issue makes mach emit a
genuine PIE Mach-O executable so the darwin self-host runs on Apple Silicon.

The investigation (#1718) already prototyped the cheap parts and ruled out the
expensive alternatives — keep raw svc (no libSystem syscall routing), classic
LC_DYLD_INFO rebase (no chained fixups), zero dylib dependency. The prototypes live
on branch invel/1718-scope (env-gated LC_MAIN/LC_BUILD_VERSION/LC_UUID/MH_PIE
toggles in emit_dyn_exec); use them as the starting point.

Scope:

• Writer (src/lang/target/of/macho.mach): emit a position-independent image —
  MH_PIE header flag, a relocatable layout (PIE base, not the current fixed
  0xfffff000), an LC_MAIN entry (replacing LC_UNIXTHREAD for the darwin main
  executable), LC_BUILD_VERSION, LC_UUID, and an LC_DYLD_INFO(_ONLY) carrying a
  rebase stream — classic rebase opcodes (REBASE_OPCODE_*) covering every
  in-image absolute pointer, so dyld slides them by the load bias. No imports ⇒ no
  dylibs / stubs / GOT / bind stream. Keep the ad-hoc LC_CODE_SIGNATURE (codegen/link: emit ad-hoc Mach-O code signature so Darwin executables run on Apple Silicon #1679) and
  the header-in-__TEXT layout (codegen/link(macho): static executables map the mach header outside __TEXT → SIGKILL on Apple Silicon #1717).
• Linker (src/lang/be/linker.mach): collect the in-image absolute relocations
  (global pointers, vtable function pointers, absolute data relocs) into rebase
  records for the writer. arm64 TEXT is already PC-relative; the rebase work is in
  absolute DATA pointers. This is the dominant cost of the issue.
• Route the compiler's darwin self-host through this PIE writer (it has no dynamic
  imports, so the dynamic path collapses to "PIE + rebase, zero dylibs").
• Pairs with the mach-std LC_MAIN entry change (briar-systems/mach-std, the epic: dynamic libSystem-linked darwin executables (Apple Silicon rejects static images) #1718
  child) — the register-convention entry the PIE LC_MAIN jumps to.

Acceptance

• A mach-built aarch64-darwin PIE executable execs on a macos-14 runner (no
  Killed: 9); the native a→b→c darwin fixpoint converges and mach test . passes
  on Apple Silicon. Validate via infra: CD-authored darwin release build (x86_64 + aarch64, native fixpoint) #1680's workflow_dispatch.
• The rebase stream is well-formed (llvm-objdump/otool: MH_PIE, LC_MAIN,
  LC_DYLD_INFO rebase opcodes resolve to the right addresses); the ad-hoc signature
  still validates.
• x86_64-darwin, linux, and windows output unaffected; the linux self-host fixpoint
  stays byte-identical and mach test . green.