Implement a thread pool

Implement a thread pool

This adds a simple thread pool to gdb.  In the end, this seemed
preferable to the approach taken in an earlier version of this series;
namely, starting threads in the parallel-foreach implementation.  This
approach reduces the overhead of starting new threads, and also lets
the user control (in a subsequent patch) exactly how many worker
threads are running.

gdb/ChangeLog
2019-11-26  Christian Biesinger  <cbiesinger@google.com>
	    Tom Tromey  <tom@tromey.com>

	* gdbsupport/thread-pool.h: New file.
	* gdbsupport/thread-pool.c: New file.
	* Makefile.in (COMMON_SFILES): Add thread-pool.c.
	(HFILES_NO_SRCDIR): Add thread-pool.h.

Change-Id: I597bb642780cb9d578ca92373d2a638efb44fe52