Silence crate external span error in x86 platforms

This causes issues in at least `dist-i586-gnu-i586-i686-musl`,
possibly others.

src/test/ui/in-band-lifetimes/mismatched_trait_impl-2.rs

@@ -1,3 +1,5 @@
+// ignore-x86
+// ^ due to stderr output differences
 use std::ops::Deref;
 trait Trait {}
 

src/test/ui/in-band-lifetimes/mismatched_trait_impl-2.stderr

@@ -1,5 +1,5 @@
 error: `impl` item signature doesn't match `trait` item signature
-  --> $DIR/mismatched_trait_impl-2.rs:8:5
+  --> $DIR/mismatched_trait_impl-2.rs:10:5
    |
 LL |     fn deref(&self) -> &dyn Trait {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found fn(&Struct) -> &dyn Trait