mips64 / mips64el CI: spurious FPU exception flags under qemu-user

[ViralBShah]

The mips64 and mips64el entries in .github/workflows/cross.yml are disabled because the in-tree test suite fails under qemu-user on Ubuntu 24.04. Disabled in #346.

Symptom

make test (which runs test/test-float and test/test-double) reports ~1600 failures, all of the form:

```
Failure: yn (1, 1.5) == -0.41230862697391129595: Exception "Invalid operation" set
Failure: yn (1, 2.0) == -0.10703243154093754689: Exception "Divide by zero" set
```

The computed value is correct in each case — the test fails only because the FPU exception flag is unexpectedly set after the call. No numeric disagreement.

Failing run (mips64): https://github.com/JuliaMath/openlibm/actions/runs/27987699090/job/82832967366
Failing run (mips64el): https://github.com/JuliaMath/openlibm/actions/runs/27987699090/job/82832967387

32-bit mips and mipsel pass under the same qemu / toolchain combination, which strongly suggests this is qemu-user mishandling MIPS64 FPU status register semantics rather than an openlibm bug. Possibly related: qemu's softfloat IEEE flag mapping for the n64 ABI.

To investigate

• Reproduce on bare metal or under qemu-system-mips64 (full-system) and compare flag behavior.
• Check whether qemu-user >= <some version> fixes it (current pin is Ubuntu 24.04's qemu).
• If qemu-user is unfixable here, decide whether to keep mips64 CI entirely or run only with exception-flag assertions skipped (the openlibm-test suite has SKIP_FP_EXCEPT_TEST=1; the in-tree suite would need an equivalent knob).

How to re-enable

In .github/workflows/cross.yml, uncomment the two matrix entries:

```yaml

• { arch: mips64, triple: mips64-linux-gnuabi64 }
• { arch: mips64el, triple: mips64el-linux-gnuabi64 }
  ```

[ViralBShah]

Tested in PR #346 — bumping the runner to ubuntu-26.04 (qemu 10.2.1) does not work: Ubuntu 26.04 dropped gcc-mips64-linux-gnuabi64 from the apt feed entirely. Only binutils-mips64-linux-gnuabi64-dbg remains; no GCC cross-compiler. This is consistent with Debian's broader MIPS deprecation (mips64el is a Debian Ports architecture now, not first-class).

State of the matrix:

• 24.04 has the toolchain but qemu 8.2 → spurious FPU exception flags (this issue).
• 26.04 has new qemu but no toolchain.
• 25.10 (questing) still has both, but is non-LTS / EOL July 2026.

Realistic paths forward, in order of effort:

1. Live with mips64 disabled. Re-evaluate annually.
2. Install qemu-user 10.x from 25.10 (or a PPA) on the 24.04 runner before building. Confirms the qemu hypothesis at the cost of one fragile cross-suite apt step.
3. Build inside a debian:trixie container (which still has the cross-gcc) on a 24.04 runner with newer qemu. Most involved; survives longest.