[OpenMP][libomptarget] Notify the plugins regarding new mapping/unmappings

The NextGen plugins use the information regarding new mapping/unmappings to
lock/unlock the corresponding host buffer and speed up the host-device memory
transfers involving those buffers. The locking/unlocking is disabled by default
and can be enabled by the LIBOMPTARGET_LOCK_MAPPED_HOST_BUFFERS envar. The
envar accepts boolean values (on/off) and a special option:
  - off:       Do not lock mapped host buffers (default).
  - on:        Lock mapped host buffers automatically, but do not report lock
               failures if the plugin fails to lock them.
  - mandatory: Lock mapped host buffers automatically and treat locking failures
               in the plugins as fatal errors. This option may be useful for
               debugging purposes.

Differential Revision: https://reviews.llvm.org/D142514