Align IntelGOTPCRel4 with R_X86_64_GOTPCREL.

Add an addend field to reloc_external, and use it to move the
responsibility for accounting for the difference between the end of an
instruction (where the PC is considered to be in PC-relative on intel)
and the beginning of the immediate field into the encoding code.

Specifically, this makes IntelGOTPCRel4 directly correspond to
R_X86_64_GOTPCREL, instead of also carrying an implicit `- 4`.

cranelift/src/filetest/binemit.rs

@@ -72,13 +72,29 @@ impl binemit::CodeSink for TextSink {
         write!(self.text, "{}({}) ", reloc, ebb_offset).unwrap();
     }
 
-    fn reloc_external(&mut self, reloc: binemit::Reloc, name: &ir::ExternalName) {
+    fn reloc_external(
+        &mut self,
+        reloc: binemit::Reloc,
+        name: &ir::ExternalName,
+        addend: binemit::Addend,
+    ) {
         write!(
             self.text,
-            "{}({}) ",
+            "{}({}",
             reloc,
             name,
         ).unwrap();
+        if addend != 0 {
+            write!(
+                self.text,
+                "{:+}",
+                addend,
+            ).unwrap();
+        }
+        write!(
+            self.text,
+            ") ",
+        ).unwrap();
     }
 
     fn reloc_jt(&mut self, reloc: binemit::Reloc, jt: ir::JumpTable) {