x64: refactor REX-specific encoding machinery to its own module

In preparation for adding new encoding modes to the x64 backend (e.g. VEX,
EVEX), this change moves all of the current instruction encoding functions to
`encodings::rex`. This refactor does not change any logic.

cranelift/codegen/src/isa/x64/inst/mod.rs

@@ -22,6 +22,7 @@ pub mod args;
 mod emit;
 #[cfg(test)]
 mod emit_tests;
+pub(crate) mod encoding;
 pub mod regs;
 pub mod unwind;