Cmm: declare CPU requirements on RTS procedures

[angerman]

Stacked on #196; supersedes #194. Backports GHC !16687 with stable-branch corrections and stronger regressions.

RTS vector procedures need AVX2/AVX-512 while scalar and V16 procedures must retain the baseline instruction set. Instead of matching source filenames in Cabal/Hadrian, procedures now declare their requirements with Cmm __attribute__((target("avx2"))) attributes. Apply/Jumps routines remain generated and compiled into the RTS at build time, preserving the existing linking arrangement.

Implementation and corrections

• Carry attributes through Cmm lowering, deterministic renaming, procedure splitting and raw conversion.
• Apply native backend settings per procedure during selection, register allocation, spills and printing. Adapt to stable's separate AVX feature fields and optional backend guards.
• Preserve the complete LLVM target-default/global feature list and last-occurrence override ordering. Honor LLVM-version gates, including AVX-512 policy; retain OpenBSD retpolines and Android defaults.
• Annotate vector apply/save and Jumps procedures under their register-availability guards. Remove per-file ISA build rules; scalar/V16 and unsupported/unregisterised targets retain baseline behavior.
• Reject unknown or unsupported attributes. Correct the upstream test output/platform guards, strengthen LLVM output normalization and add an API regression independent of external LLVM executables.
• Preserve Fix compiler and RTS portability issues exposed by haskell.nix #196's PIC trampoline/CFG fix while adapting its procedure metadata to RawCmmProcInfo. Retain the shared build/CI prerequisites from Fix compiler and RTS portability issues exposed by haskell.nix #196.

The attribute is additive; global RTS flags must still avoid enabling AVX for baseline procedures. It does not add runtime CPU dispatch.

Validation

The clean history is 93c3dc1716 → 09f5c34611 → 0aae70d65d (#196) → 0aaa5fa085 (#197). The feature patch is unchanged by this restacking; it now includes #196's reviewed Windows-fixture, ranlib (including resolved PATH-launcher diagnostics) and pinned-mirror bootstrap corrections. CI has restarted on both exact heads. Fast-forward only after the current candidates are green. The abandoned link-time implementation is absent.

• Astra independently reviewed the source, stable-branch corrections, integration and shared CI repairs. Its compiled-test configuration finding is fixed.
• The combined source previously built and linked a standalone compiler from its full 871-module dependency closure. All 11 attribute/backend/LLVM probes and four actual PIC assembly/lint cases passed. Synthetic cross settings qualify code generation, not target ABI/runtime; local boot-library validation does not replace full bootstrap/bindist CI.
• Previous head 5e64db50c7 passed normal CI and Nix CI: all four normal suites, six Nix native suites, three MULTI cross builds and three strict artifact-consuming e2e jobs. Linux JUnits explicitly confirm the Cmm attribute, isolation/rejection and LLVM API regressions. The e2e jobs cover native/Wasm/JS hello execution and the Miso Wasm build. These are prior-head results; current CI must pass again.
• Both complete Windows runtime suites subsequently passed at heads 5fa0da603b / f2956276fe. The shared external Wasm bootstrap failures are repaired with a pinned, checksum-verified official mirror archive, retaining full upstream setup; focused failure-path and real Darwin SDK/libffi probes pass, and Astra cleared the repair. Complete setup and consumers still require new-head CI.
• The shared Windows failure is isolated and the corrected fixture passes all original assertions in the native A/B diagnostic. The Intel suite subsequently exposed a bare-command/resolved-path mismatch in that filter. The corrected PATH-aware filter has a failing-before/passing-after real-process regression, actual cctools launcher reproduction and independent Astra POSIX/Windows probes; rebuilt Intel static001 remains current CI qualification.
• The previous Intel release suite also timed out compiling T26537 with -O2 -fregs-graph. A focused same-runner comparison passed the original Fix compiler and RTS portability issues exposed by haskell.nix #196 and Cmm: declare CPU requirements on RTS procedures #197 compilers in 497s and 402s with unchanged flags, outputs and 1,500-second limits. The timeout did not reproduce; one ordered pair establishes no general speedup or flakiness classification. A subsequent ARM dynamic suite timed out in the same test, while its static counterpart passed; current full platform suites remain required.

LLVM API execution is distinct from LLVM Cmm emission; actual LLVM emission was exercised locally with external LLVM tools. No timeout, assertion or expected result has been relaxed.

A previous Ubuntu 24 runtime suite (20c3c72df9) reported one T21651(threaded1) segmentation fault; the same test passed both normal Linux configurations. Its targeted retry is retained without changing assertions or classifying the crash. Current candidate platform suites must pass.