Don't use _Atomic for jobs sometimes...

Don't use _Atomic for jobs sometimes...

The use of _Atomic leads to really bad code generation in the compiler
(on x86, you get 2 "mfence" memory barriers around each access with gcc8, despite
x86 being ordered and cache coherent). But there's a fallback in the code that
just uses volatile which is more than plenty in practice.

If we're nervous about cross thread synchronization for these variables, we should
make the YIELD function be a compiler/memory barrier instead.

performance before (after last commit)

   Matrix          SGEMM cycles    MPC                                   DGEMM cycles      MPC
  48 x 48               10630.0   10.6       0.7%                             18112.8      6.2      -0.7%
  64 x 64               20374.8   13.0       1.9%                             40487.0      6.5       0.4%
  65 x 65              141955.2    1.9    -428.3%                            146708.8      1.9    -179.2%
  80 x 80              178921.1    2.9    -369.6%                            186032.7      2.8    -156.6%
  96 x 96              205436.2    4.3    -233.4%                            224513.1      3.9     -97.0%
 112 x 112             244408.2    5.8    -162.7%                            262158.7      5.4     -47.1%
 128 x 128             321334.5    6.5    -141.3%                            333829.0      6.3     -29.2%

Performance with this patch (roughly a 2x improvement):

   Matrix          SGEMM cycles    MPC                                   DGEMM cycles      MPC
  48 x 48               10756.0   10.5      -0.5%                             18296.7      6.1      -1.7%
  64 x 64               20490.0   12.9       1.4%                             40615.0      6.5       0.0%
  65 x 65               83528.3    3.3    -210.9%                             96319.0      2.9     -83.3%
  80 x 80              101453.5    5.1    -166.3%                            128021.7      4.0     -76.6%
  96 x 96              149795.1    5.9    -143.1%                            168059.4      5.3     -47.4%
 112 x 112             191481.2    7.3    -105.8%                            204165.0      6.9     -14.6%
 128 x 128             265019.2    7.9     -99.0%                            272006.4      7.7      -5.3%