clover: Enable cl_khr_fp64 for devices that support doubles v4

clover: Enable cl_khr_fp64 for devices that support doubles v4

v2:
  - Report correct values for CL_DEVICE_NATIVE_VECTOR_WIDTH_DOUBLE
    and CL_DEVICE_PREFERRED_VECTOR_WIDTH_DOUBLE.
  - Only define cl_khr_fp64 if the extension is supported.
  - Remove trailing space from extension string.
  - Rename device query function from cl_khr_fp64() to
    has_doubles().

v3:
  - Return 0 for device::doubled_fp_confg() when doubles aren't
    supported.

v4:
  - Remove device query for double fp_config.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>