Add a method to indicate section address re-assignment is finished.

Prior to this patch RuntimeDyld attempted to re-apply relocations every time reassignSectionAddress was called (via MCJIT::mapSectionAddress).  In addition to being inefficient and redundant, this led to a problem when a section was temporarily moved too far away from another section with a relative relocation referencing the section being moved.  To fix this, I'm adding a new method (finalizeObject) which the client can call to indicate that it is finished rearranging section addresses so the relocations can safely be applied.

llvm-svn: 167400