perf thread: Make thread__find_symbol() return the symbol searched

perf thread: Make thread__find_symbol() return the symbol searched

Instead of just returning it in al.sym, allowing for some simplification
in its users, and to make it consistent with thread__find_map().

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-4axi2sigslffdixzxbehvgoj@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>