Repin maintenance submodules for Linx bring-up

[zhoubot]

Summary

• repin the dirty maintenance submodules to their review branches for the current Linx bring-up pass
• register tools/LinxCoreModel in .gitmodules and pin it to the merged upstream main SHA
• keep the root PR narrow to gitlinks plus submodule registration only

Included repins

• compiler/llvm -> 68cb581ab36520a31638ae018c7c65cbac8ecb3f (#14)
• emulator/qemu -> 70bd94ca3fe8c4df640396a3f66bcdede8c5ca20 (#24)
• kernel/linux -> cfa703586f3ff85006e613bb712b2c25aca1df76 (#18)
• lib/glibc -> a9b2c7146df3e26a767b00d3a1e41aafca9a8d85 (#8)
• skills/linx-skills -> a7f4e780ad88522197decae66bc3235db8bca752 (#26)
• tools/pyCircuit -> 59a46833e10bfdabe2f108c6721eb6a2e4e8e0f5 (#36)
• tools/LinxCoreModel -> 6cd2790a10af1e46a293ab85ec4fc75ed35234f9 (merged LinxCoreModel#1)

Validation

• bash tools/ci/check_repo_layout.sh
• git diff --cached --check
• module-local validation is recorded in the linked leaf PRs:
  • LLVM: targeted Linx CodeGen/MC checks over the modified llc/clang/llvm-mc tests
  • QEMU: ninja -C build qemu-system-linx64, ISA coverage report, opcode-sync audit
  • Linux: check_linx_virt_defconfig_spec.py, ctx_tq_irq_smoke.py
  • glibc: GLIBC_G1B_ALLOW_BLOCKED=1 bash lib/glibc/tools/linx/build_linx64_glibc_g1b.sh
  • pyCircuit / skills: module-specific validation scripts recorded in their PRs