Replace size_t members with uint64_t and pointer members (never used as
pointers, but instead as unique idenitifiers) with int64_t. This makes
the structs identical between 32-bit and 64-bit systems.
On 64-bit bit systems, the ABI is maintained. On 32-bit systems, this
is an ABI breaking change. The ABI was previously broken in r315662.
This also imposes a small API change on userspace consumers who must
handle kernel pointers becoming virtual addresses.
PR: 228301 (exp-run by antoine)