[v3] mutex duplicate removal

[Benjamin Kosnik]

This merges various mutex code scattered around the library into
ext/concurrence.h. In doing so, all duplicate effort has been
eliminated, and mutexes construction and initialization should now be
done in a consistent way throughout the library.

Two include files were moved from bits to ext, since they are
extensions and already in namespace __gnu_cxx, and should have been in
this include directory from the beginning.

I think some of the loops for threaded code could become simpler. I
believe with -02, gcc can now optimize

if (0)
 do_this;
else
  do_that;

to:
  do_that;

Since __gthread_active_p() is 0 on systems without defined(__GTHREADS)
the obvious cleanup (not attempted) could be applied.

-benjamin

tested x86/linux
tested x86/linux --disable-threads
tested x86/linux abi

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