samples/bpf: build with -D__TARGET_ARCH_$(SRCARCH)

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: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>