PPC: Reimplement pooled data reference calculation (#167)

* PPC: Calculate pooled relocations

Reimplements #140

The relocations are now generated when the object is first read in `parse`, right after the real relocations are read.

`resolve_relocation` was changed to take `obj.symbols` instead of `obj` as an argument, because `obj` itself doesn't exist yet at the time the relocations are being read.

* Improve readability of PPC pool relocs code

* Fix regression causing extern pool relocs to be ignored

* Fix showing incorrect diff when diffing weak stripped symbol with an addend

This is a regression that was introduced by #158 diffing addends in addition to symbol names. But it's not really a bug in that PR, rather it seems like I simply never added the offset into the addend when creating a fake pool relocation for an extern symbol. So this commit fixes that root issue instead.

* Add PPC "Calculate pooled data references" option

* Fix objdiff-wasm compilation errors

* Update PPC test snapshots

objdiff-core/src/diff/code.rs

@@ -502,17 +502,3 @@ fn diff_instruction(
 
     Ok(InstructionDiffResult::new(InstructionDiffKind::None))
 }
-
-// TODO
-// fn find_symbol_matching_fake_symbol_in_sections(
-//     fake_symbol: &ObjSymbol,
-//     sections: &[ObjSection],
-// ) -> Option<ObjSymbol> {
-//     let orig_section_index = fake_symbol.orig_section_index?;
-//     let section = sections.iter().find(|s| s.orig_index == orig_section_index)?;
-//     let real_symbol = section
-//         .symbols
-//         .iter()
-//         .find(|s| s.size > 0 && (s.address..s.address + s.size).contains(&fake_symbol.address))?;
-//     Some(real_symbol.clone())
-// }