[Libomptarget] Move API implementations into GenericPluginTy (#86683)

Summary:
The plan is to remove the entire plugin interface and simply use the
`GenericPluginTy` inside of `libomptarget` by statically linking against
it. This means that inside of `libomptarget` we will simply do
`Plugin.data_alloc` without the dynamically loaded interface. To reduce
the amount of code required, this patch simply moves all of the RTL
implementation functions inside of the Generic device. Now the
`__tgt_rtl_` interface is simply a shallow wrapper that will soon go
away. There is some redundancy here, this will be improved later. For
now what is important is minimizing the changes to the API.