test: codegen: skip catch-unwind on riscv64

It isn't clear to me if this is a bug or not, hence the FIXME

src/test/codegen/catch-unwind.rs

@@ -1,5 +1,14 @@
 // compile-flags: -O
 
+// On x86 the closure is inlined in foo() producting something like
+// define i32 @foo() [...] {
+// tail call void @bar() [...]
+// ret i32 0
+// }
+// On riscv the closure is another function, placed before fn foo so CHECK can't
+// find it
+// ignore-riscv64 FIXME
+
 #![crate_type = "lib"]
 
 extern "C" {