[lld] handle typeZeroFill atoms in ELF/Native/YAML

BSS atoms dont take any file space in the Input file. They are associated
with a contentType(typeZeroFill). Similiar zero fill types also exist which
have the same meaning in terms of occupying file space in the Input.

These atoms have to be handled seperately when writing to the
lld's intermediate file or the lld test infrastructure.

Also adds a test.
[lld][ELF] Rename typeTLV content type for ELF

typeTLV content type is used by Darwin to represent thread local
storage. A new contentType has to be made to represent ELF
thread local storage data. These have been set to

- typeThreadZeroFill (represents TBSS storage)
- typeThreadData     (represents TDATA storage)
[lld][ELF][Cleanup] Section associated with the relocation traversed

There is no change in functionality, this uses the defined way to access
the relocation section that belongs to a particular section.
[lld][ELF] Dont ignore zero sized sections.

There may be relocations that may be pointing to the section
even if the section sizes are 0. We shouldnot ignore them
for that regard.