selftests: bpf: extend sub-register mode compilation to all bpf object files

At the moment, we only do extra sub-register mode compilation on bpf object
files used by "test_progs". These object files are really loaded and
executed.

This patch further extends sub-register mode compilation to all bpf object
files, even those without corresponding runtime tests. Because this could
help testing LLVM sub-register code-gen, kernel bpf selftest has much more
C testcases with reasonable size and complexity compared with LLVM
testsuite which only contains unit tests.

There were some file duplication inside BPF_OBJ_FILES_DUAL_COMPILE which
is removed now.

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>