This patch introduces parallelization of DWARF generation on a per
compilation unit basis. Each compilation unit now operates on a
separate set of symbols, so it's safe to send each compilation unit to
a goroutine to be processed in parallel.
Doing this requires some restructing to ensure that any new symbols
needed are created up front, since we can't create any new syms during
the parallel portion. Similarly, the parallel portion can't set any
symbol attributes, so the check that verifies we haven't doubly listed
any DIE syms had to be reworked, and setting of reachability has to be
delayed until after the parallel phase is complete.