Implement emitting Windows unwind information for fastcall functions. (#1155)

* Implement emitting Windows unwind information for fastcall functions.

This commit implements emitting Windows unwind information for x64 fastcall
calling convention functions.

The unwind information can be used to construct a Windows function table at
runtime for JIT'd code, enabling stack walking and unwinding by the operating
system.

* Address code review feedback.

This commit addresses code review feedback:

* Remove unnecessary unsafe code.
* Emit the unwind information always as little endian.
* Fix comments.

A dependency from cranelift-codegen to the byteorder crate was added.
The byteorder crate is a no-dependencies crate with a reasonable
abstraction for writing binary data for a specific endianness.

* Address code review feedback.

* Disable default features for the `byteorder` crate.
* Add a comment regarding the Windows ABI unwind code numerical values.
* Panic if we encounter a Windows function with a prologue greater than 256
  bytes in size.

cranelift/codegen/src/context.rs

@@ -193,6 +193,16 @@ impl Context {
         sink.info
     }
 
+    /// Emit unwind information.
+    ///
+    /// Requires that the function layout be calculated (see `relax_branches`).
+    ///
+    /// Only some calling conventions (e.g. Windows fastcall) will have unwind information.
+    /// This is a no-op if the function has no unwind information.
+    pub fn emit_unwind_info(&self, isa: &dyn TargetIsa, mem: &mut Vec<u8>) {
+        isa.emit_unwind_info(&self.func, mem);
+    }
+
     /// Run the verifier on the function.
     ///
     /// Also check that the dominator tree and control flow graph are consistent with the function.