selftests/bpf: compile progs with -D__TARGET_ARCH_$(SRCARCH)

This opens up the possibility of accessing registers in an
arch-independent way.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>