clover: Add CL_MEM_HOST_* flag checks.

clover: Add CL_MEM_HOST_* flag checks.

Those flags have been introduced in OpenCL 1.2.

[ Francisco Jerez: Rebase.  Throw CL_INVALID_VALUE from
  clCreateSubBuffer if the subbuffer drops access flags from its
  parent.  Use single function taking the set of allowed host access
  flags to validate memory transfer operands. ]

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