selftests/bpf: Do not update vmlinux.h unnecessarily

%.bpf.o objects depend on vmlinux.h, which makes them transitively
dependent on unnecessary libbpf headers. However vmlinux.h doesn't
actually change as often.

When generating vmlinux.h, compare it to a previous version and update
it only if there are changes.

Example of build time improvement (after first clean build):
  $ touch ../../../lib/bpf/bpf.h
  $ time make -j8
Before: real  1m37.592s
After:  real  0m27.310s

Notice that %.bpf.o gen step is skipped if vmlinux.h hasn't changed.
Signed-off-by: Ihor Solodrai <ihor.solodrai@pm.me>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/CAEf4BzY1z5cC7BKye8=A8aTVxpsCzD=p1jdTfKC7i0XVuYoHUQ@mail.gmail.com
Link: https://lore.kernel.org/bpf/20240828174608.377204-2-ihor.solodrai@pm.me

tools/testing/selftests/bpf/Makefile

@@ -399,10 +399,14 @@ $(HOST_BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile)		       \
 		    DESTDIR=$(HOST_SCRATCH_DIR)/ prefix= all install_headers
 endif
 
+# vmlinux.h is first dumped to a temprorary file and then compared to
+# the previous version. This helps to avoid unnecessary re-builds of
+# $(TRUNNER_BPF_OBJS)
 $(INCLUDE_DIR)/vmlinux.h: $(VMLINUX_BTF) $(BPFTOOL) | $(INCLUDE_DIR)
 ifeq ($(VMLINUX_H),)
 	$(call msg,GEN,,$@)
-	$(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $@
+	$(Q)$(BPFTOOL) btf dump file $(VMLINUX_BTF) format c > $(INCLUDE_DIR)/.vmlinux.h.tmp
+	$(Q)cmp -s $(INCLUDE_DIR)/.vmlinux.h.tmp $@ || mv $(INCLUDE_DIR)/.vmlinux.h.tmp $@
 else
 	$(call msg,CP,,$@)
 	$(Q)cp "$(VMLINUX_H)" $@