perf btf: Make the sigtrap test helper to find a member by name widely available

By introducing a tools/perf/util/btf.c to collect utilities not yet
available via libbpf, the first being a way to find a member by name
once we get the type_id for the struct.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>