[Libomptarget] Replace global `PluginTy::get` interface with references (#86595)

Summary:
We have a plugin singleton that implements the Plugin interface. This
then spawns separate device and kernels. Previously when these needed to
reach into the global singleton they would use the `PluginTy::get`
routine to get access to it. In the future we will move away from this
as the lifetime of the plugin will be handled by `libomptarget`
directly. This patch removes uses of this inside of the plugin
implementaion themselves by simply keeping a reference to the plugin
inside of the device.

The external `__tgt_rtl` functions still use the global method, but will
be removed later.