The runner.o may start building before libbpf headers are installed,
and as a result build fails. This happened a couple of times on
libbpf/ci test jobs:
  * https://github.com/libbpf/ci/actions/runs/11447667257/job/31849533100
  * https://github.com/theihor/libbpf-ci/actions/runs/11445162764/job/31841649552

Headers are installed in a recipe for $(BPFOBJ) target, and adding an
order-only dependency should ensure this doesn't happen.

Signed-off-by: Ihor Solodrai <ihor.solodrai@pm.me>
Signed-off-by: Tejun Heo <tj@kernel.org>

The sched_ext selftests is missing proper cross-compilation support, a
proper target entry, and out-of-tree build support.

When building the kselftest suite, e.g.:

  make ARCH=riscv CROSS_COMPILE=riscv64-linux-gnu-  \
    TARGETS=sched_ext SKIP_TARGETS="" O=/output/foo \
    -C tools/testing/selftests install

or:

  make ARCH=arm64 LLVM=1 TARGETS=sched_ext SKIP_TARGETS="" \
    O=/output/foo -C tools/testing/selftests install

The expectation is that the sched_ext is included, cross-built, the
correct toolchain is picked up, and placed into /output/foo.

In contrast to the BPF selftests, the sched_ext suite does not use
bpftool at test run-time, so it is sufficient to build bpftool for the
build host only.

Add ARCH, CROSS_COMPILE, OUTPUT, and TARGETS support to the sched_ext
selftest. Also, remove some variables that were unused by the
Makefile.

Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: David Vernet <void@manifault.com>
Tested-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

Add basic selftests.

Signed-off-by: David Vernet <dvernet@meta.com>
Acked-by: Tejun Heo <tj@kernel.org>