[Libomptarget] Add an external interface to dynamic shared memory

This patch adds an external interface to access the dynamic shared
memory buffer in the device runtime. The function introduced is
``llvm_omp_get_dynamic_shared``. This includes a host-side
definition that only returns a null pointer so that it can be used when
host-fallback is enabled without crashing. Support for dynamic shared
memory was also ported to the old device runtime.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D110957