[greatbridf]

Summary

The x86_64 CI leg never got past setup: first Setup QEMU failed with
E: Unable to locate package qemu-system-x86_64, and once that is fixed the
build fails in build/mbr.bin with error: no such command: objcopy. Both
are missing CI dependencies, not kernel problems. The loongarch64 leg is
left exactly as it is on master (still failing its build).

Root cause

1. Wrong QEMU package name. The workflow derives the package from the
   matrix arch, qemu-system-${{ matrix.arch }}. That only works for
   riscv64 by accident: there is no qemu-system-riscv64 package, but
   qemu-system-misc Provides: that name, so apt resolves it. For x86_64
   the generated name qemu-system-x86_64 is provided by nothing —
   qemu-system-x86 only Provides: qemu-system-amd64,
   qemu-system-x86-64 and qemu-kvm (hyphen, not underscore). apt aborts,
   and Configure, build, artifact upload and the test run are all skipped.
2. Missing cargo objcopy. With QEMU available, the image build reaches
   make build/mbr.bin, which runs cargo objcopy. That subcommand comes
   from cargo-binutils, which CI never installs. The riscv64 leg never needed
   it because it boots the ELF kernel with -kernel.

Fix

• Add an explicit qemu_pkg to each matrix entry (qemu-system-riscv64 for
  riscv64, qemu-system-x86 for x86_64) and install
  ${{ matrix.qemu_pkg || 'qemu-system-misc' }}. The || default keeps the
  loongarch64 leg installing a QEMU capable of running loongarch64 (as on
  master, where the alias resolved to qemu-system-misc) without touching
  that entry.
• Install cargo-binutils before the build; llvm-tools (needed by
  cargo-objcopy) is already installed by the preceding step.
• qemu-kvm is dropped from the install line: it only resolved as a
  Provides alias of qemu-system-x86, and the test step already forces
  QEMU_ACCEL: '', so KVM is never used.

The arch: [riscv64, loongarch64] matrix list is unchanged.

Verification

• Ubuntu 24.04 noble main/binary-amd64 package metadata:
  • qemu-system-x86: Provides: qemu-kvm, qemu-system-amd64, qemu-system-any, qemu-system-i386, qemu-system-x86-64 — no
    qemu-system-x86_64.
  • qemu-system-misc: Provides: ... qemu-system-loongarch64, qemu-system-riscv64 ....
• Run 40545656339 (master): riscv64 passed Setup QEMU, x86_64 failed at
  Setup QEMU.
• Run for this branch at c2c1db0 (QEMU fix only): x86_64 Setup QEMU
  succeeded, and the build failed at build/mbr.bin with
  error: no such command: objcopy — this PR's second commit.
• Run 40549445351 at b543cae (this branch, both commits):
  • build-and-run-test (riscv64) — success
  • build-and-run-test (x86_64, qemu-system-x86, x86_64-unknown-none, acpi_compat) — success: Setup QEMU, build, artifact upload and
    Test 7185 with ARCH=x86_64 passed all green
  • build-and-run-test (loongarch64) — still fails, at the same build step as
    on master, on eonix_hal errors (unresolved imports eonix_mm::paging::{NoAlloc, Page}, E0437/E0050/E0046 on
    RawTrapContext/TrapReturn). That is the pre-existing leg state and is
    untouched by this PR.
• fdisk (/usr/sbin/fdisk), mkfs.fat and killall are present on the
  runner, as the successful riscv64 run shows.
• No local build was run for this change (CI-configuration only); the runs on
  this PR are the verification.

Fixes: 731798d ("pipeline: remove loongarch64 and add x86 build and test")