Currently, the `humility diagnose` command contains its own code for
looking up the task table. This code only uses the `TASK_TABLE_BASE`
symbol used by older kernels, and does not look for the
`TASK_TABLE_SPACE` symbol used by newer kernels. This means that on a
recent kernel, the `diagnose` command will fail:

```console
$ humility -t gimletlet diagnose
humility: attached to 0483:3754:000B00154D46501520383832 via ST-Link V3

--- Initial Inspection ---

Taking initial snapshot of task status...
humility diagnose failed: expected symbol TASK_TABLE_BASE not found

```

This commit changes `humility diagnose` to use the
`HubrisArchive::task_table` method to look up the task table, instead.
Unlike the previous code, this method will look for both the
`TASK_TABLE_BASE` and `TASK_TABLE_SIZE` symbols of older kernels, _and_
the `TASK_TABLE_SPACE` symbol of newer kernels. Now, it works for my
recent Hubris build:

```console
$ humility -t gimletlet diagnose
humility: attached to 0483:3754:000B00154D46501520383832 via ST-Link V3

--- Initial Inspection ---

Taking initial snapshot of task status...
System last rebooted 791183 ticks ago; assuming tick=millisecond, 791.183s ago
Snapshot taken, 25 tasks in application.

--- Tasks: First Pass ---

Checking for any suspicious task attributes...

--- Advancing Time ---

Resuming core for a bit...
Core halted after 1001 more ticks.

--- Tasks: Second Pass ---

--- Generating Coredump ---

humility: dumping to hubris.core.0
humility: dumped 731.46KB in 29 seconds
Leaving CPU halted in case you want to inspect.

--- End Of Report ---

```

Fixes #483

the error from `task_table` is actually already more descriptive