fix: save LR in arm64 non-G0 asmcall trampoline (root-cause of darwin/arm64 hang)

[lirenjie95]

What

Root-caused and fixed the darwin/arm64 asmcall non-G0 trampoline hang found by task 4.3 CI (TestCallFuncP0 timing out on macos-latest), and extended CI so all supported OS/ISA combos get real trampoline coverage.

Root cause

The arm64 ASMCALL macro was CALL R8; RET — it never saved LR (x30). Unlike amd64, where return addresses live on the stack, on arm64 BLR R8 overwrites x30 with the trampoline's own return address (CallFuncP*+8). The C callee (a leaf) returns correctly to +8, but the trampoline's own RET then jumps to x30 — which still holds +8 — and spins forever jumping to itself.

Reproduced locally on macOS arm64 and confirmed with lldb: the hung thread's pc == lr == CallFuncP0+8 (the RET instruction), process state R (spinning, not blocked).

This is an arm64 ISA bug, not darwin-specific: linux/arm64 would hang identically — it was simply never covered (CI only ran the tests on linux/amd64). The G0 variants passed because G0ASMCALL already saves/restores x30. SP alignment, PAC/arm64e, and stack protectors were all ruled out.

Changes

• asmcall/arm64.s: ASMCALL now saves/restores x30 around the call (and 16-byte-aligns SP per AAPCS), mirroring the G0 variant. amd64 is untouched.
• asmcall/calltest/call_test.go, asmcall/calltest/calltest.go: build tags relaxed to (linux || darwin || windows) && (amd64 || arm64); comments updated with the root cause. macOS CI now runs the trampoline tests instead of compile/vet-only.
• .github/workflows/ci.yml:
  • added an ubuntu-24.04-arm leg — the arm64 LR bug class was invisible to linux/amd64;
  • added a windows-latest (amd64) leg — amd64.s has a Win64 calling-convention path (args in CX/DX/R8 via GOOS_windows) no leg ever executed. mem-ring is unix-only and test/go depends on it, so this leg runs go test ./asmcall/... only; the runner image's MSYS2 mingw-w64 gcc covers the cgo-built calltest callees.
• docs/ci.md: documented the new legs.

This supersedes the previously considered fallback of restricting the asm fast path to Linux — macOS and Windows keep the fast path.

Testing

• Local macOS arm64 (Go 1.18.1): go test ./asmcall/... passes, including all 8 trampoline variants on darwin (previously hanging).
• Stress: 200k iterations of CallFuncP{0..3} with GC()/Gosched() interleaved — pass.
• gofmt -l clean; go vet -unsafeptr=false ./... clean.
• Cross-compile checked: linux/arm64, linux/amd64, windows/amd64 all build.
• The new ubuntu-24.04-arm, windows-latest and updated macos-latest legs validate on this PR's CI run.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.09%. Comparing base (7208be8) to head (380d22c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #193   +/-   ##
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  Coverage   97.09%   97.09%           
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  Files          24       24           
  Lines        3955     3955           
=======================================
  Hits         3840     3840           
  Misses        115      115           

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