selftests/bpf: Ensure no DWARF relocations for BPF object files

Add -mattr=dwarfris attribute to llc to avoid having relocations against DWARF
data. These relocations make it impossible to inspect DWARF contents: all
strings are invalid.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191121070743.1309473-2-andriin@fb.com

tools/testing/selftests/bpf/Makefile

@@ -161,7 +161,7 @@ $(OUTPUT)/flow_dissector_load.o: flow_dissector_load.h
 define CLANG_BPF_BUILD_RULE
 	($(CLANG) $3 -O2 -target bpf -emit-llvm				\
 		-c $1 -o - || echo "BPF obj compilation failed") | 	\
-	$(LLC) -march=bpf -mcpu=probe $4 -filetype=obj -o $2
+	$(LLC) -mattr=dwarfris -march=bpf -mcpu=probe $4 -filetype=obj -o $2
 endef
 # Similar to CLANG_BPF_BUILD_RULE, but with disabled alu32
 define CLANG_NOALU32_BPF_BUILD_RULE