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/ir/function.rs

@@ -83,6 +83,11 @@ pub struct Function {
     /// Track the original source location for each instruction. The source locations are not
     /// interpreted by Cranelift, only preserved.
     pub srclocs: SourceLocs,
+
+    /// Instruction that marks the end (inclusive) of the function's prologue.
+    ///
+    /// This is used for some calling conventions to track the end of unwind information.
+    pub prologue_end: Option<Inst>,
 }
 
 impl Function {
@@ -104,6 +109,7 @@ impl Function {
             offsets: SecondaryMap::new(),
             jt_offsets: SecondaryMap::new(),
             srclocs: SecondaryMap::new(),
+            prologue_end: None,
         }
     }
 
@@ -123,6 +129,7 @@ impl Function {
         self.offsets.clear();
         self.jt_offsets.clear();
         self.srclocs.clear();
+        self.prologue_end = None;
     }
 
     /// Create a new empty, anonymous function with a Fast calling convention.