[ELF] Remove dead variable.
[ELF] Change Segment to hold Chunks instead of Sections. No functionality change.
[ELF] Emit the file header and program headers at the beginning of the first segment.

This is required for the loader to correctly set AT_PHDR.

With this lld can correctly link against dietlibc and get to main.
[lld] Add -emit-yaml option. This outputs yaml instead of a binary.
[ELF] Properly handle constant data.

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