selftests: bpf: makefile support sub-register code-gen test mode

This patch enables testing some eBPF programs under sub-register
compilation mode.

Only enable this when there is BPF_JMP32 support on both LLVM and kernel.
This is because only after BPF_JMP32 added, code-gen for complex program
under sub-register mode will be clean enough to pass verification.

This patch splits TEST_GEN_FILES into BPF_OBJ_FILES and
BPF_OBJ_FILES_DUAL_COMPILE. The latter are those objects we would like to
compile for both default and sub-register mode. They are also objects used
by "test_progs".

Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>