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

[kledzik at apple.com]

One area of linking not fleshed out in lld is the "canBeNullAtBuildtime" attribute for UndefinedSymbols.  That was intended to model ELF weak undefines.  Is that how __tls_get_addr works with gnu ld?  If so, then getting canBeNullAtBuildtime to work in general could solve this specific issue without needing to special case  __tls_get_addr.

http://reviews.llvm.org/D5505