Refactor AArch64 ABI support to extract common bits for shared impl with x64.

We have observed that the ABI implementations for AArch64 and x64 are
very similar; in fact, x64's implementation started as a modified copy
of AArch64's implementation. This is an artifact of both a similar ABI
(both machines pass args and return values in registers first, then the
stack, and both machines give considerable freedom with stack-frame
layout) and a too-low-level ABI abstraction in the existing design. For
machines that fit the mainstream or most common ABI-design idioms, we
should be able to do much better.

This commit factors AArch64 into machine-specific and
machine-independent parts, but does not yet modify x64; that will come
next.

This should be completely neutral with respect to compile time and
generated code performance.

cranelift/codegen/src/machinst/mod.rs

@@ -123,12 +123,16 @@ pub mod blockorder;
 pub use blockorder::*;
 pub mod abi;
 pub use abi::*;
+pub mod abi_impl;
+pub use abi_impl::*;
 pub mod pretty_print;
 pub use pretty_print::*;
 pub mod buffer;
 pub use buffer::*;
 pub mod adapter;
 pub use adapter::*;
+pub mod helpers;
+pub use helpers::*;
 
 /// A machine instruction.
 pub trait MachInst: Clone + Debug {