selftests/bpf: get rid of -D__x86_64__

-D__x86_64__ workaround was used to make /usr/include/features.h
to follow expected path through the system include headers.
This is not portable.
Instead define dummy stubs.h which is used by 'clang -target bpf'

Fixes: 6882804c916b ("selftests/bpf: add a test for overlapping packet range checks")
Signed-off-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/bpf/Makefile

···
 CLANG ?= clang
 
 %.o: %.c
-	$(CLANG) -I../../../include/uapi -I../../../../samples/bpf/ \
-		-D__x86_64__ -Wno-compare-distinct-pointer-types \
+	$(CLANG) -I. -I../../../include/uapi -I../../../../samples/bpf/ \
+		-Wno-compare-distinct-pointer-types \
 		-O2 -target bpf -c $< -o $@

tools/testing/selftests/bpf/gnu/stubs.h

···
+/* dummy .h to trick /usr/include/features.h to work with 'clang -target bpf' */