Support R_MIPS_GPREL16 relocations correctly (#88)

* Support R_MIPS_GPREL16 relocations correctly

symbols defined in the same file require adding a
special ri_gp_value from the .reginfo section to
their relocation calculations.

* Run nightly rustfmt

* Prevent potential panic when slicing .reginfo

objdiff-core/src/arch/mod.rs

@@ -1,7 +1,7 @@
 use std::{borrow::Cow, collections::BTreeMap};
 
 use anyhow::{bail, Result};
-use object::{Architecture, Object, ObjectSymbol, Relocation, RelocationFlags, Symbol};
+use object::{Architecture, File, Object, ObjectSymbol, Relocation, RelocationFlags, Symbol};
 
 use crate::{
     diff::DiffObjConfig,
@@ -28,8 +28,13 @@ pub trait ObjArch: Send + Sync {
         config: &DiffObjConfig,
     ) -> Result<ProcessCodeResult>;
 
-    fn implcit_addend(&self, section: &ObjSection, address: u64, reloc: &Relocation)
-        -> Result<i64>;
+    fn implcit_addend(
+        &self,
+        file: &File<'_>,
+        section: &ObjSection,
+        address: u64,
+        reloc: &Relocation,
+    ) -> Result<i64>;
 
     fn demangle(&self, _name: &str) -> Option<String> { None }