selftests/bpf: Fix runqslower cross-endian build

selftests/bpf: Fix runqslower cross-endian build

The runqslower binary from a cross-endian build currently fails to run
because the included skeleton has host endianness. Fix this by passing the
target BPF endianness to the runqslower sub-make.

Fixes: 5a63c33d6f00 ("selftests/bpf: Support cross-endian building")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250125071423.2603588-1-itugrok@yahoo.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>