bpf: Add resolve_btfids tool to resolve BTF IDs in ELF object

The resolve_btfids tool scans elf object for .BTF_ids section
and resolves its symbols with BTF ID values.

It will be used to during linking time to resolve arrays of BTF
ID values used in verifier, so these IDs do not need to be
resolved in runtime.

The expected layout of .BTF_ids section is described in main.c
header. Related kernel changes are coming in following changes.

Build issue reported by 0-DAY CI Kernel Test Service.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200711215329.41165-2-jolsa@kernel.org

tools/bpf/resolve_btfids/Build

@@ -0,0 +1,10 @@
+resolve_btfids-y += main.o
+resolve_btfids-y += rbtree.o
+resolve_btfids-y += zalloc.o
+resolve_btfids-y += string.o
+resolve_btfids-y += ctype.o
+resolve_btfids-y += str_error_r.o
+
+$(OUTPUT)%.o: ../../lib/%.c FORCE
+	$(call rule_mkdir)
+	$(call if_changed_dep,cc_o_c)