[ORC] Add ELFNixPlatform::Create overload -- Pass ORC runtime as def generator.

The existing Create method took a path to the ORC runtime and created a
StaticLibraryDefinitionGenerator for it. The new overload takes a
std::unique_ptr<DefinitionGenerator> directly instead. This provides more
flexibility when constructing MachOPlatforms. E.g. The runtime archive can be
embedded in a special section in the ORC controller executable or library,
rather than being on-disk.

This is the ELFNixPlatform equivalent of the MachOPlatform change in
be2fc577.