util/disk_cache: hash pointer size and gpu name into cache keys

util/disk_cache: hash pointer size and gpu name into cache keys

This allows to get rid of the arch and gpu name directories.

v2: (Timothy Arceri) don't use an opaque data type to store
    pointer size and gpu name.

v3: (Timothy Arceri) use blob to store driver keys just make sure
    to store null terminator for strings, and make sure blob is
    defined by disk_cache and not it's users.

v4: (Timothy Arceri) fix typo, and make ptr_size a uint8_t.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>