make procedure DynArrayAssign public
## Summary
Delphi publishes procedure DynArrayAssign(var Dest: Pointer; Source: Pointer; typeInfo: Pointer);
FPC has this procedure as fpc_dynarray_assign but it is marked as compiler only.
Some generic code requires access to DynArrayAssign so it would be good to make this function public instead of having to re-implement it in user library/code.
## System Information
- **Operating system:** Windows
- **Processor architecture:** x86-64
- **Device:** Computer
## Relevant 3rd party information
https://docwiki.embarcadero.com/Libraries/Sydney/en/System.DynArrayAssign
issue