perf thread: Introduce thread__find_map()

perf thread: Introduce thread__find_map()

Out of thread__find_add_map(..., 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_map() 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-q27xee34l4izpfau49w103s6@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>