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/unwind.rs

@@ -2,16 +2,22 @@
 #[cfg(feature = "enable-serde")]
 use serde::{Deserialize, Serialize};
 
+#[cfg(feature = "unwind")]
 pub mod systemv;
+
+#[cfg(feature = "unwind")]
 pub mod winx64;
 
 /// Represents unwind information for a single function.
 #[derive(Clone, Debug, PartialEq, Eq)]
 #[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))]
+#[non_exhaustive]
 pub enum UnwindInfo {
     /// Windows x64 ABI unwind information.
+    #[cfg(feature = "unwind")]
     WindowsX64(winx64::UnwindInfo),
     /// System V ABI unwind information.
+    #[cfg(feature = "unwind")]
     SystemV(systemv::UnwindInfo),
 }