[gdb/build] Fix build with undefined CXX_STD_THREAD

[gdb/build] Fix build with undefined CXX_STD_THREAD

When building gdb on openSUSE Leap 42.3, we trigger the case that
CXX_STD_THREAD is undefined, and run into:
...
gdb/maint.c: In function 'void maintenance_show_worker_threads \
  (ui_file*, int, cmd_list_element*, const char*)':
gdb/maint.c:877:14: error: 'gdb::thread_pool' has not been declared
         gdb::thread_pool::g_thread_pool->thread_count ());
              ^
Makefile:1647: recipe for target 'maint.o' failed
make[1]: *** [maint.o] Error 1
...

Fix this by handling the undefined CXX_STD_THREAD case in
maintenance_show_worker_threads, such that we get:
...
$ gdb -q -batch -ex "maint show worker-thread"
The number of worker threads GDB can use is 0.
...

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28312