[PATCH] [lld] [ELF] Support for general dynamic TLS relocations on X86_64

[kledzik at apple.com]

>to complete linking an executable, all symbols must be resolved and we cannot finish it with undefined references

The way linking should work is that if any UndefinedAtoms remain, if all of them are marked canBeNullAtBuildtime, the LinkingContext should instantiate SharedLibraryAtoms to resolve those.  canBeNullAtBuildtime is something only ELF supports, so the LinkingContext needs to called to do this.

With the functionality in place, the trick is to create the UndefinedAtoms for tls_get_addr as canBeNullAtBuildtime.

http://reviews.llvm.org/D5505