cranelift-codegen: Prepare cranelift codegen for usage from Winch (#5413)

This commit prepares the x64 pieces from cranelift codegen to be consumed by
Winch for binary emission. This change doesn't introduce or modifies
functionality it makes the necessary pieces for binary emission public.

This change also improves documentation where applicable.

cranelift/codegen/src/lib.rs

@@ -89,7 +89,10 @@ pub mod write;
 
 pub use crate::entity::packed_option;
 pub use crate::machinst::buffer::{MachCallSite, MachReloc, MachSrcLoc, MachStackMap, MachTrap};
-pub use crate::machinst::{CompiledCode, TextSectionBuilder};
+pub use crate::machinst::{
+    CompiledCode, Final, MachBuffer, MachBufferFinalized, MachInst, MachInstEmit, Reg,
+    TextSectionBuilder, Writable,
+};
 
 mod alias_analysis;
 mod bitset;