anv: Align anv_descriptor_pool::host_mem
Otherwise anv_descriptor_set is accessed through an unaligned pointer, which is undefined behavior in C.
anv_descriptor_set.c:1620:17: runtime error: member access within misaligned address 0x61900002c2b5
for type 'struct anv_descriptor_set', which requires 8 byte alignment 0x61900002c2b5
Fixes: 2570a58b ("anv: Implement descriptor pools")