Reorganize target plugins

Tasks

• Review OpenMP target API and interface with the Clang compiler
• Review CUDA plugin
• Review AMDGPU plugin (WIP)
• Review Johannes cleanup patch (WIP)
• Fix locking issues
  • Some critical regions are unnecessarily large
  • std::mutex is used instead of a faster spinlock on small critical operations
• Fix or mitigate dynamic memory issues
  • Kernel launches define multiple std::vectors

This re-organization focuses on the CUDA and AMDGPU plugins, but it should into account the VE and GENERIC plugins too.