Instead of checking for stage1 at every callsite, move the logic
inside `allocateAtom`. This is fine since this logic will disappear
anyhow once I add expanding and shifting segments and sections.
This commit makes it possible to combine self-hosted with a pre-compiled
C object file, e.g.:
```
zig-out/bin/zig build-exe hello.zig add.o
```
where `add.o` is a pre-compiled C object file.
instead of globally storing unresolved and tentative defs,
store indices to actual symbols in the functions that are responsible
for symbol resolution.
instead, pass it in functions that require it. Also, when parsing
relocs, make Object part of the context struct where we pass in
additional goodies such as `*MachO` or `*Allocator`.
then, when sorting sections within segments, clear and redo the
ordinals since we re-apply them to symbols anyway. It is vital
to have the ordinals consistent with parsing and resolving relocs
however.
Thanks to this, we no longer need to do allocs per symbol name
landing in the symbol resolver, plus we do not need to actively
track if the string was already inserted into the string table.
It makes sense to have them as a dependent type since they only ever
deal with TextBlocks. Simplify Relocations to rely on symbol indices
and symbol resolver rather than pointers.
