Fix PR libgomp/66761 and PR libgomp/67303 (tsan warnings)

[VandeVondele Joost]

The following fixes two warnings reported by tsan when running OMP'ed code. As suggested by Andrew Pinski in PR67303 for gomp_iter_guided_next, by employing a relaxed atomic load. The same pattern occurred a couple of more times, so fixed as well. I used the same approach for the warning in do_spin (PR66761), even though this is a different context.

Bootstrapped and regtested on x86_64-unknown-linux-gnu for trunk, and manually verified that the PR is fixed using a tsan compiled libgomp based on gcc-5.1.

OK for trunk ? 
This applies without changes to the 5 branch, should I commit there as well ?

libgomp/ChangeLog:

2015-08-22  Joost VandeVondele  <vondele@gnu.gcc.org>

        PR libgomp/66761
        PR libgomp/67303
        * iter.c (gomp_iter_dynamic_next): Employ an atomic load.
        (gomp_iter_guided_next): Idem.
        * iter_ull.c (gomp_iter_ull_dynamic_next): Idem.
        (gomp_iter_ull_guided_next): Idem.
        * config/linux/wait.h (do_spin): Idem.
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