Ignore system_error in thread startup

Ignore system_error in thread startup

libstdc++ might change so that it always implements std::thread, but
then have thread startup simply fail.  This is being discussed here:

https://gcc.gnu.org/pipermail/gcc-patches/2020-November/558736.html

This patch pre-emptively changes gdb to handle this scenario.  It
seemed fine to me to ignore all system errors at thread startup, so
that is what this does.

gdbsupport/ChangeLog
2020-11-20  Tom Tromey  <tromey@adacore.com>

	* thread-pool.cc (thread_pool::set_thread_count): Ignore system
	errors.