clover: try userptr for CL_MEM_USE_HOST_PTR

clover: try userptr for CL_MEM_USE_HOST_PTR

According to spec, CL_MEM_USE_HOST_PTR should directly use host memory,
if possible. This is just what userptr is for, so use it.

In case the memory cannot be mapped, a fallback similar to
CL_MEM_COPY_HOST_PTR is used.

v2: constify, drop unneeded cast

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