druntime implementation for the new ELF registration …

2nd commit of #395.

- Store thread local list of self-registering
  libraries. The compiler provides ModuleInfos
  and EH tables.

- Use dl_iterate_phdr to get further information
  from the runtime linker.

- Iterate over program headers
  to find writeable segments, i.e. the ones with
  GC roots. The program header also provides
  TLS module indices and the TLS segment size.