Almost 60% performance gain on GCN for sha512crypt.

Recommended for (at least) 12.08 Catalyst driver.

From:
  Local work size (LWS) 64, global work size (GWS) 16384
  Benchmarking: sha512crypt (rounds=5000) [OpenCL]... DONE
  Raw:	8316 c/s real, 409600 c/s virtual

To:
  Local work size (LWS) 64, global work size (GWS) 32768
  Benchmarking: sha512crypt (rounds=5000) [OpenCL]... DONE
  Raw:	13159 c/s real, 819200 c/s virtual