Bug in work-stealing dequeu (possible memory write access reordering in pushWSDeque)

[Karel Gardas]

Hello,

while working on ARM port, I've found bug in WSDeque.c which might 
possibly also hurt other architectures -- depending on how weak is their 
memory model (i.e. how much load/store reordering it permits).

I'm certainly not sure if it affects x86, but I'm not sure if PowerPC is 
safe here, hence I'm writing this.

For fix preview and explanation see: 
https://github.com/kgardas/ghc/commit/ac7c59eba9806fc8216f44342c12b784e6680483

Thanks,
Karel