gdb: fix build errors in gdbsupport/thread-pool.h used with old gcc

gdb: fix build errors in gdbsupport/thread-pool.h used with old gcc

When I build gdb with gcc 8.3, there exist the following build errors,
rename the typedef to task_t to fix them.

  CXX      thread-pool.o
In file included from /home/loongson/gdb.git/gdbsupport/thread-pool.cc:21:
/home/loongson/gdb.git/gdbsupport/../gdbsupport/thread-pool.h: In member function ‘std::future<void> gdb::thread_pool::post_task(std::function<void()>&&)’:
/home/loongson/gdb.git/gdbsupport/../gdbsupport/thread-pool.h:69:44: error: declaration of ‘task’ shadows a previous local [-Werror=shadow=local]
     std::packaged_task<void ()> task (std::move (func));
                                            ^~~~
/home/loongson/gdb.git/gdbsupport/../gdbsupport/thread-pool.h:102:39: note: shadowed declaration is here
   typedef std::packaged_task<void ()> task;
                                       ^~~~
/home/loongson/gdb.git/gdbsupport/../gdbsupport/thread-pool.h: In member function ‘std::future<_Res> gdb::thread_pool::post_task(std::function<T()>&&)’:
/home/loongson/gdb.git/gdbsupport/../gdbsupport/thread-pool.h:80:41: error: declaration of ‘task’ shadows a previous local [-Werror=shadow=local]
     std::packaged_task<T ()> task (std::move (func));
                                         ^~~~
/home/loongson/gdb.git/gdbsupport/../gdbsupport/thread-pool.h:102:39: note: shadowed declaration is here
   typedef std::packaged_task<void ()> task;
                                       ^~~~

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>