selftests/bpf: Enable test test_progs-cpuv4 for gcc build kernel

Currently, test_progs-cpuv4 is generated with clang build kernel
when bpf cpu=v4 is supported by the clang compiler.
Let us enable test_progs-cpuv4 for gcc build kernel as well.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230728055745.2285202-1-yonghong.song@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>