[ELF2] - Linker script EXTERN command implemented.

The reason of collecting all undefines in vector is that during reading files we already need to have Symtab created. Or like was done in that patch - to put undefines from scripts somewhere to delay Symtab.addUndefinedOpt() call.

Differential Revision: http://reviews.llvm.org/D13870

llvm-svn: 250711