tlskey.m: Use hashtable from runtime on AmigaOS

tlskey.m used OFMapTable only on AmigaOS. This became a problem when
autorelease pools were moved into the runtime, as autorelease pools use
TLS. The build then broke, as there suddenly was a dependency from the
runtime on ObjFW.

This now uses the hashtable from the runtime and also no longer uses
OFList, thus fixing the build. As we always use the runtime on AmigaOS
anyway, this is fine.