[ELF] - Move members of LinkerScript to LinkerScriptBase. NFC.

That moves all members that s possible to move for now (all which
does not depend on ELFT templating).

After that change LinkerScript contains only 8 methods in total,
and I believe it is possible to move them all after tweaking other
parts of linker. And we will be able to have single class for 
linkerscript at the end.

llvm-svn: 297735