[dev.link] cmd/link: revise representation of dwarfp
Change linker DWARF generation to move away from emitting a single
giant list of DWARF symbols, and instead emit a list of descriptors,
with each descriptor holding the symbols for a specific DWARF section.
While placing all DWARF symbols in a single lists does come in handy
in certain instances, it also creates a lot of confusion and weird
code in other cases, specifically where we want to perform operations
on a section-by-section basis (resulting in code that tries to
re-discover section boundaries by walking/inspecting the list).