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/x64/inst/unwind.rs

@@ -0,0 +1,36 @@
+use super::Inst;
+use crate::isa::unwind::UnwindInfo;
+use crate::machinst::{UnwindInfoContext, UnwindInfoGenerator, UnwindInfoKind};
+use crate::result::CodegenResult;
+
+#[cfg(feature = "unwind")]
+pub use self::systemv::create_cie;
+
+#[cfg(feature = "unwind")]
+mod systemv;
+
+pub struct X64UnwindInfo;
+
+impl UnwindInfoGenerator<Inst> for X64UnwindInfo {
+    #[allow(unused_variables)]
+    fn create_unwind_info(
+        context: UnwindInfoContext<Inst>,
+        kind: UnwindInfoKind,
+    ) -> CodegenResult<Option<UnwindInfo>> {
+        // Assumption: RBP is being used as the frame pointer for both calling conventions
+        // In the future, we should be omitting frame pointer as an optimization, so this will change
+        Ok(match kind {
+            #[cfg(feature = "unwind")]
+            UnwindInfoKind::SystemV => {
+                const WORD_SIZE: u8 = 8;
+                systemv::create_unwind_info(context, WORD_SIZE)?.map(UnwindInfo::SystemV)
+            }
+            #[cfg(feature = "unwind")]
+            UnwindInfoKind::Windows => {
+                //TODO winx64::create_unwind_info(context)?.map(|u| UnwindInfo::WindowsX64(u))
+                panic!();
+            }
+            UnwindInfoKind::None => None,
+        })
+    }
+}