The basic cl_mem buffer API doesn't enable access to the underlying raw
pointers in the device memory, preventing its use in host side
data structures that need pointer references to objects.
This API adds a minimal increment on top of cl_mem that provides such
capabilities.

The version 0.1.0 is implemented in PoCL and rusticl for prototyping,
but everything's still up for discussion. chipStar is the first client
that uses the API.

Co-authored-by: Sun Serega <sunserega2@gmail.com>

Changed the CL_MEM_DEVICE_ADDRESS_EXT wording for multi-device
cases "all", not "any", covering a case where not all devices
can ensure the same address across the context. In that case
CL_INVALID_VALUE can be returned.  Defined sub-buffer address
computation to be 'base_addr + origin'. Added error conditions
for clSetKernelExecInfo when the device doesn't support
device pointers.

...and renamed them to CL_MEM_DEVICE_SHARED_ADDRESS_EXT and
CL_MEM_DEVICE_PRIVATE_ADDRESS_EXT. The first one guarantees the
same address across all devices in the context, whereas the latter
allows per-device addresses.