Change how we compute offsets with linker scripts.

This fixes pr30367, but more importantly, it changes how we compute offsets.

Now offset computation in a walk over linker script commands, like the
rest of assignAddresses. IMHO this is simpler to understand and if we
ever have to create multiple outputsections or chunks to change how we
handle test/ELF/linkerscript/alternate-sections.s it should be easier
to do it.

llvm-svn: 281736