[OpenMP][libomptarget] Add AMDGPU NextGen plugin with asynchronous behavior

This commit adds the AMDGPU NextGen plugin inheriting from PluginInterface's classes.
It also implements the asynchronous behavior in the plugin operations: kernel launches
and memory transfers. To this end, it implements the concept of streams of asynchronous
operations. The streams are implemented using the HSA signals to define input and output
dependencies between asynchronous operations.

Missing features:
  - Retrieve the maximum number of threads per group that a kernel can run. This requires
    reading the image.
  - Implement __tgt_rtl_sync_event, not used on the libomptarget side.

Differential Revision: https://reviews.llvm.org/D138389