relibc: replace 2 panic-site unimplemented!() in dynamic linker with proper errors

Two catch-all branches in the dynamic linker panicked the process if
an executable used a relocation type not handled by the explicit match
arms. Both functions are Result-returning, but the catch-all used
unimplemented!() which expands to panic!() — silently killing the
process instead of returning Err.

  * ld_so/dso.rs:1129 (static_relocate): the
    _ => unimplemented!("relocation type {:?}", reloc.kind)
    arm panics for any relocation not in the match. Now returns
    Err(format!("unsupported relocation type {:?}", reloc.kind)).
    Process startup with an unfamiliar relocation type now fails
    gracefully at dlopen time instead of aborting.

  * ld_so/dso.rs:1203 (lazy_relocate): same pattern in the
    (reloc.kind, resolve) match. Now returns
    Err(format!("unsupported relocation type {:?} with resolve {:?}",
                reloc.kind, resolve)).

This is the third round of relibc panic-site fixes (after RTLD_NOLOAD|
RTLD_NOW and readdir_r). Found by the Round 10 audit
(local/docs/3D-DESKTOP-COMPREHENSIVE-PLAN.md §10).

src/ld_so/dso.rs

@@ -1126,7 +1126,9 @@ impl DSO {
             RelocationKind::TLSDESC => {
                 self.do_tlsdesc_reloc(reloc, ptr.cast::<usize>(), global_scope)
             }
-            _ => unimplemented!("relocation type {:?}", reloc.kind),
+            _ => {
+                return Err(format!("unsupported relocation type {:?}", reloc.kind));
+            }
         }
 
         Ok(())
@@ -1200,11 +1202,11 @@ impl DSO {
                 }
 
                 _ => {
-                    unimplemented!(
-                        "relocation type {:?} with resolve {:?}",
+                    return Err(format!(
+                        "unsupported relocation type {:?} with resolve {:?}",
                         reloc.kind,
                         resolve
-                    )
+                    ));
                 }
             }
         }