clover/device: Calculate CL_DEVICE_MEM_BASE_ADDR_ALIGN in device

clover/device: Calculate CL_DEVICE_MEM_BASE_ADDR_ALIGN in device

The CL CTS queries CL_DEVICE_MEM_BASE_ADDR_ALIGN for a device and
then allocates user pointers aligned to that value for its tests.

The minimum value is defined as:
  the size (in bits) of the largest OpenCL built-in data type supported
  by the device (long16 in FULL profile, long16 or int16 in EMBEDDED
  profile) for devices that are not of type CL_DEVICE_TYPE_CUSTOM.

At the moment, all known devices that support user pointers require
CPU page alignment for buffers created from user pointers, so just
query that from sysconf.

v3: Use std::max instead of MAX2 (Francisco)
    Add missing unistd include
v2: Use system page size instead of a new pipe cap

Signed-off-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by (v2): Jan Vesely <jan.vesely@rutgers.edu>