perf thread: Introduce thread__find_symbol()

Out of thread__find_addr_location(..., MAP__FUNCTION, ...), idea here is to
continue removing references to MAP__{FUNCTION,VARIABLE} ahead of
getting both types of symbols in the same rbtree, as various places do
two lookups, looking first at MAP__FUNCTION, then at MAP__VARIABLE.

So thread__find_symbol() will eventually do just that, and 'struct
symbol' will have the symbol type, for code that cares about that.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-n7528en9e08yd3flzmb26tth@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>