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: 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>

samples/bpf/Makefile

@@ -284,7 +284,7 @@ $(obj)/%.o: $(src)/%.c
 	$(Q)$(CLANG) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(EXTRA_CFLAGS) -I$(obj) \
 		-I$(srctree)/tools/testing/selftests/bpf/ \
 		-D__KERNEL__ -D__BPF_TRACING__ -Wno-unused-value -Wno-pointer-sign \
-		-D__TARGET_ARCH_$(ARCH) -Wno-compare-distinct-pointer-types \
+		-D__TARGET_ARCH_$(SRCARCH) -Wno-compare-distinct-pointer-types \
 		-Wno-gnu-variable-sized-type-not-at-end \
 		-Wno-address-of-packed-member -Wno-tautological-compare \
 		-Wno-unknown-warning-option $(CLANG_ARCH_ARGS) \