samples/bpf: Use own EXTRA_CFLAGS for clang commands

It can overlap with CFLAGS used for libraries built with gcc if
not now then in next patches. Correct it here for simplicity.
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20191011002808.28206-5-ivan.khoronzhuk@linaro.org

samples/bpf/Makefile

@@ -218,10 +218,10 @@ BTF_LLVM_PROBE := $(shell echo "int main() { return 0; }" | \
 			  /bin/rm -f ./llvm_btf_verify.o)
 
 ifneq ($(BTF_LLVM_PROBE),)
-	EXTRA_CFLAGS += -g
+	BPF_EXTRA_CFLAGS += -g
 else
 ifneq ($(and $(BTF_LLC_PROBE),$(BTF_PAHOLE_PROBE),$(BTF_OBJCOPY_PROBE)),)
-	EXTRA_CFLAGS += -g
+	BPF_EXTRA_CFLAGS += -g
 	LLC_FLAGS += -mattr=dwarfris
 	DWARF2BTF = y
 endif
@@ -280,8 +280,9 @@ $(obj)/hbm_edt_kern.o: $(src)/hbm.h $(src)/hbm_kern.h
 # useless for BPF samples.
 $(obj)/%.o: $(src)/%.c
 	@echo "  CLANG-bpf " $@
-	$(Q)$(CLANG) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(EXTRA_CFLAGS) -I$(obj) \
-		-I$(srctree)/tools/testing/selftests/bpf/ -I$(srctree)/tools/lib/bpf/ \
+	$(Q)$(CLANG) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(BPF_EXTRA_CFLAGS) \
+		-I$(obj) -I$(srctree)/tools/testing/selftests/bpf/ \
+		-I$(srctree)/tools/lib/bpf/ \
 		-D__KERNEL__ -D__BPF_TRACING__ -Wno-unused-value -Wno-pointer-sign \
 		-D__TARGET_ARCH_$(SRCARCH) -Wno-compare-distinct-pointer-types \
 		-Wno-gnu-variable-sized-type-not-at-end \