samples: bpf: Fix cross-compiling error by using bootstrap bpftool

Currently, when cross compiling bpf samples, the host side cannot
use arch-specific bpftool to generate vmlinux.h or skeleton. Since
samples/bpf use bpftool for vmlinux.h, skeleton, and static linking
only, we can use lightweight bootstrap version of bpftool to handle
these, and it's always host-native.
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220714024612.944071-2-pulehui@huawei.com

samples/bpf/Makefile

@@ -282,12 +282,10 @@ $(LIBBPF): $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile) | $(LIBBPF_OU
 
 BPFTOOLDIR := $(TOOLS_PATH)/bpf/bpftool
 BPFTOOL_OUTPUT := $(abspath $(BPF_SAMPLES_PATH))/bpftool
-BPFTOOL := $(BPFTOOL_OUTPUT)/bpftool
-$(BPFTOOL): $(LIBBPF) $(wildcard $(BPFTOOLDIR)/*.[ch] $(BPFTOOLDIR)/Makefile) | $(BPFTOOL_OUTPUT)
+BPFTOOL := $(BPFTOOL_OUTPUT)/bootstrap/bpftool
+$(BPFTOOL): $(wildcard $(BPFTOOLDIR)/*.[ch] $(BPFTOOLDIR)/Makefile) | $(BPFTOOL_OUTPUT)
 	$(MAKE) -C $(BPFTOOLDIR) srctree=$(BPF_SAMPLES_PATH)/../../ 		\
-		OUTPUT=$(BPFTOOL_OUTPUT)/ \
-		LIBBPF_OUTPUT=$(LIBBPF_OUTPUT)/ \
-		LIBBPF_DESTDIR=$(LIBBPF_DESTDIR)/
+		OUTPUT=$(BPFTOOL_OUTPUT)/ bootstrap
 
 $(LIBBPF_OUTPUT) $(BPFTOOL_OUTPUT):
 	$(call msg,MKDIR,$@)