[lld] Add -emit-yaml option. This outputs yaml instead of a binary.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@171710 91177308-0d34-0410-b5e6-96231b3b80d8
[ELF] Properly handle constant data.

With this hello world works with dietlibc on x86-64 Linux.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@171711 91177308-0d34-0410-b5e6-96231b3b80d8