1.0.42.34: threads: Consolidate thread-specific static symbols

1.0.42.34: threads: Consolidate thread-specific static symbols

  * The x86, x86-64 and ppc backends all had the same three static
symbols used for threading.  Rather than having each backend add
the same symbols to the static symbol list, add them to the list
of common static symbols.  In the unlikely(?) event that we commit
a threaded port to mainline that doesn't use these symbols, we can
revisit this change.