machinst x64: New backend unwind (#2266)

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

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

@@ -0,0 +1,15 @@
+use super::*;
+use crate::isa::unwind::UnwindInfo;
+use crate::result::CodegenResult;
+
+pub struct AArch64UnwindInfo;
+
+impl UnwindInfoGenerator<Inst> for AArch64UnwindInfo {
+    fn create_unwind_info(
+        _context: UnwindInfoContext<Inst>,
+        _kind: UnwindInfoKind,
+    ) -> CodegenResult<Option<UnwindInfo>> {
+        // TODO
+        Ok(None)
+    }
+}