[Libomptarget][NFC] Move global Libomptarget state to a struct
Presently, there a number of global variables in libomptarget (devices,
RTLs, tables, mutexes, etc.) that are not placed within a struct. This
patch places them into a struct `PluginManager`. All of the functions
that act on this data remain free.
Differential Revision: https://reviews.llvm.org/D90519