While $ARCH can be relatively flexible (see Makefile and
tools/scripts/Makefile.arch), $SRCARCH always corresponds to a directory
name under arch/.

Therefore, build samples with -D__TARGET_ARCH_$(SRCARCH), since that
matches the expectations of bpf_helpers.h.
Signed-off-by: NIlya Leoshkevich <iii@linux.ibm.com>
Acked-by: NVasily Gorbik <gor@linux.ibm.com>
Acked-by: NAndrii Nakryiko <andriin@fb.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>