Request flushing denorms to zero in OpenCL

Request flushing denorms to zero in OpenCL

This change adds by default the -cl-denorms-are-zero to the flags used
for kernel compilation. This is done to:
- avoid a large performance penalty on AMD Vega with ROCm (which by
  default handles denorms on GFX9 or later).
- make the defaults uniform across CUDA and OpenCL.

Fixes #2593

Change-Id: I9e6183c4367b5960e0e21f1dd342d7695acfbc44