New --enable-threading configure option to control use of threads in GDB/GDBserver

Add the --enable-threading configure option so multithreading can be disabled
at configure time. This is useful for statically-linked builds of
GDB/GDBserver, since the thread library doesn't play well with that setup.

If you try to run a statically-linked GDB built with threading, it will crash
when setting up the number of worker threads.

This new option is also convenient when debugging GDB in a system with lots of
threads, where the thread discovery code in GDB will emit too many messages,
like so:

[New Thread 0xfffff74d3a50 (LWP 2625599)]

If you have X threads, that message will be repeated X times.

The default for --enable-threading is "yes".