machinst x64: New backend unwind (#2266)

Addresses unwind for experimental x64 backend. The preliminary code enables backtrace on SystemV call convension.

cranelift/codegen/src/machinst/abi.rs

@@ -155,6 +155,9 @@ pub trait ABICallee {
         from_slot: SpillSlot,
         ty: Option<Type>,
     ) -> Self::I;
+
+    /// Desired unwind info type.
+    fn unwind_info_kind(&self) -> UnwindInfoKind;
 }
 
 /// Trait implemented by an object that tracks ABI-related state and can