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/isa/x86/mod.rs

@@ -5,6 +5,7 @@ mod binemit;
 mod enc_tables;
 mod registers;
 pub mod settings;
+mod unwind;
 
 use super::super::settings as shared_settings;
 #[cfg(feature = "testing_hooks")]
@@ -18,6 +19,7 @@ use crate::regalloc;
 use crate::result::CodegenResult;
 use crate::timing;
 use alloc::boxed::Box;
+use alloc::vec::Vec;
 use core::fmt;
 use target_lexicon::{PointerWidth, Triple};
 
@@ -150,6 +152,13 @@ impl TargetIsa for Isa {
     fn unsigned_sub_overflow_condition(&self) -> ir::condcodes::IntCC {
         ir::condcodes::IntCC::UnsignedLessThan
     }
+
+    /// Emit unwind information for the given function.
+    ///
+    /// Only some calling conventions (e.g. Windows fastcall) will have unwind information.
+    fn emit_unwind_info(&self, func: &ir::Function, mem: &mut Vec<u8>) {
+        abi::emit_unwind_info(func, self, mem);
+    }
 }
 
 impl fmt::Display for Isa {