Add basic linker script parsing.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@176309 91177308-0d34-0410-b5e6-96231b3b80d8
[ELF][x86-64] Only emit a PLT entry for a PLT relocation if the target atom is a SharedLibraryAtom.

This seems to be what ld does, but I'm not sure how it works with symbol interposition.

With this hello-world with glibc dynamically linked works.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@176310 91177308-0d34-0410-b5e6-96231b3b80d8