threaded tests

[Loren James Rittle]

Using the standard dg- keywords, I have figured out a way to let
us test the pthread tests on selected platforms with the correct
per-port flags required.  Shall I just install them? ;-)

I have about 6 that I generated originally or readapted from GNATs PRs
(that were either tweaked off my original testing structure by others
or were posted recently enough that I will recontact the authors to
ask for permission and then give them credit).

How about if we use:

testsuite/thread/pthread[123456].cc, etc
instead of one level down so it plays nice with dg.

I know how to cover sparc-sun-solaris2, *-*-freebsd* and *-*-linux* at
first.  Others can merely add new dg- lines to support their pthread
targets.  If others want to test other thread models (gcc/gthr.h only
abstracts the mutex for thread support not the actual starting of
threads thus we can't do better yet), then they can add
thread/rtos-thread.cc or whatever.

Regards,
Loren