cranelift: refactor unwind logic to accommodate multiple backends (#2357)

*    Make cranelift_codegen::isa::unwind::input public
*    Move UnwindCode's common offset field out of the structure
*    Make MachCompileResult::unwind_info more generic
*    Record initial stack pointer offset

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

@@ -1,5 +1,5 @@
 use super::*;
-use crate::isa::unwind::UnwindInfo;
+use crate::isa::unwind::input::UnwindInfo;
 use crate::result::CodegenResult;
 
 pub struct AArch64UnwindInfo;
@@ -7,8 +7,7 @@ pub struct AArch64UnwindInfo;
 impl UnwindInfoGenerator<Inst> for AArch64UnwindInfo {
     fn create_unwind_info(
         _context: UnwindInfoContext<Inst>,
-        _kind: UnwindInfoKind,
-    ) -> CodegenResult<Option<UnwindInfo>> {
+    ) -> CodegenResult<Option<UnwindInfo<Reg>>> {
         // TODO
         Ok(None)
     }