* ci: fix Install QEMU in cross build

* ci: use qemu 9

* ci: install qemu use apt

* ci: fix QEMU_EXEC

* ci: fix clang build

One change in PR 305 was not quite right: it changed a check for whether
`$(ARCH)` is `aarch64` to a check for whether it's `aarch64 riscv64`,
which can never be true. It was a non-functional change on RISC-V (since
it can't be true) but broke AArch64, at least for FreeBSD. Notably,
changing the check to what I assume was intended, i.e. `aarch64` or
`riscv64`, breaks RISC-V. So this commit instead reverts this one change
in 305.

Move the unconditional `?=` definitions below all conditional
definitions so that conditional `?=`s can take precedence. Also forcibly
set `USEGCC=0` when `USECLANG=1`, otherwise the things guarded behind
`USEGCC=1` will be visible. Without these changes, `gmake` on FreeBSD
AArch64 incorrectly uses gcc instead of clang.

Bumps [codecov/codecov-action](https://github.com/codecov/codecov-action) from 4 to 5.
- [Release notes](https://github.com/codecov/codecov-action/releases)
- [Changelog](https://github.com/codecov/codecov-action/blob/main/CHANGELOG.md)
- [Commits](codecov/codecov-action@v4...v5)

---
updated-dependencies:
- dependency-name: codecov/codecov-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Fixes for building on AArch64 with Clang