The clearNurseries() operation resets the free pointer in each nursery
block to the start of the block, emptying the nursery.  In the
parallel GC this was done on the main GC thread, but that's bad
because it accesses the bdescr of every nursery block, and move all
those cache lines onto the CPU of the main GC thread.  With large
nurseries, this can be especially bad.  So instead we want to clear
each nursery in its local GC thread.

Thanks to Andreas Voellmy <andreas.voellmy@gmail.com> for idenitfying
the issue.

After this change and the previous patch to make the last GC a major
one, I see these results for nofib/parallel on 8 cores:

   blackscholes          +0.0%     +0.0%     -3.7%     -3.3%     +0.3%
          coins          +0.0%     +0.0%     -5.1%     -5.0%     +0.4%
           gray          +0.0%     +0.0%     -4.5%     -2.1%     +0.8%
         mandel          +0.0%     -0.0%     -7.6%     -5.1%     -2.3%
        matmult          +0.0%     +5.5%     -2.8%     -1.9%     -5.8%
        minimax          +0.0%     +0.0%    -10.6%    -10.5%     +0.0%
          nbody          +0.0%     -4.4%     +0.0%      0.07     +0.0%
         parfib          +0.0%     +1.0%     +0.5%     +0.9%     +0.0%
        partree          +0.0%     +0.0%     -2.4%     -2.5%     +1.7%
           prsa          +0.0%     -0.2%     +1.8%     +4.2%     +0.0%
         queens          +0.0%     -0.0%     -1.8%     -1.4%     -4.8%
            ray          +0.0%     -0.6%    -18.5%    -17.8%     +0.0%
       sumeuler          +0.0%     -0.0%     -3.7%     -3.7%     +0.0%
      transclos          +0.0%     -0.0%    -25.7%    -26.6%     +0.0%
--------------------------------------------------------------------------------
            Min          +0.0%     -4.4%    -25.7%    -26.6%     -5.8%
            Max          +0.0%     +5.5%     +1.8%     +4.2%     +1.7%
 Geometric Mean          +0.0%     +0.1%     -6.3%     -6.1%     -0.7%