some final thread stuff

Hi Craig,

Here are what I think will be my final pthread related patches.

The first one hopefully eliminates any build problems associated with 
top's use of the pthread library.  Now 'pthread_sigmask' will only be 
called when one or more top threads have been enabled (they're all off 
by default).  Whoever turns them on is now responsible for proper linking.

The last two patches are geared toward future multi-threaded application 
development.  Because newlib encapsulates heap allocations in some 
'__thread' classed structures, valgrind may issue some warnings. These 
patches deal with suppressing them.

I'm pretty confident our newlib is now thread safe after activating top 
thread support along with some temporary hacks.  As long as each thread 
maintains its own 'new()' context, I could find no problem with 
concurrent access.

Thanks in advance for the push.

Regards,

Jim

p.s. In the future, I resolve to pay more attention to those GitLab 
pipeline issues.