selftests: bpf: relax sub-register mode compilation criteria

Sub-register mode compilation was enabled only when there are eBPF "v3"
processor supports at both compilation time inside LLVM and runtime inside
kernel.

Given separation betwen build and test server could be often, this patch
removes the runtime support criteria.

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>