[gdb/procfs.c] Fix build failure in find_stop_signal

It fixes a regression caused by commit
1edb66d8 where thread_info::suspend was
made private.

The public thread_info API has to be used to get stop signal and avoid
build failures.

gdb/ChangeLog:

2021-07-14  Libor Bukata <libor.bukata@oracle.com>

  * gdb/procfs.c (find_stop_signal): Use thread_info API.

Change-Id: I53bc57a05cd0eca5f28ef0726d6faeeb306e7904