[TargetInfo] Undefined symbols move from ELFTargetInfo to TargetInfo

This change moves the functionality of undefinedSymbols that were created in
ELFTargetInfo into TargetInfo. Looks like MachO/Windows use similiar options
like in ELF for creating undefined symbols when the linker is invoked.

Address comment from Ruiu.

(No change in functionality, except moving the functionality from ELF to
TargetInfo, so that multiple architectures can use the same)

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@184398 91177308-0d34-0410-b5e6-96231b3b80d8