Enable back-edge CFI by default on macOS (#4720)

Also, adjust the tests that are executed on that platform. Finally,
fix a bug with obtaining backtraces when back-edge CFI is enabled.

Copyright (c) 2022, Arm Limited.

cranelift/codegen/src/isa/aarch64/inst/emit.rs

@@ -3121,6 +3121,7 @@ impl MachInstEmit for Inst {
 
                 sink.put4(0xd503233f | key << 6);
             }
+            &Inst::Xpaclri => sink.put4(0xd50320ff),
             &Inst::VirtualSPOffsetAdj { offset } => {
                 trace!(
                     "virtual sp offset adjusted by {} -> {}",