Print friendly `AT_PLATFORM` in `pauxv(1)`

Solaris `pauxv(1)` prints a human-readable name of the platform:

```
AT_SUN_PLATFORM 0xfffffc7fffdf7286 i86pc
AT_SUN_EXECNAME 0xfffffc7fffdf728c /usr/sbin/sshd
```

We could do the same for `AT_PLATFORM`. When doing so, we could probably improve our display of `AT_EXECFN` to actually try and read from memory rather than just reading `/proc/${pid}/exe`.