ARM64 backend, part 6 / 11: CLIF -> VCode<Inst> lowering.

This patch adds the lowering implementation that translates Cranelift IR
(CLIF) function bodies to VCode<Inst>, i.e., ARM64 machine instructions.

This patch contains code written by Julian Seward <jseward@acm.org> and
Benjamin Bouvier <public@benj.me>, originally developed on a side-branch
before rebasing and condensing into this patch series. See the `arm64`
branch at `https://github.com/cfallin/wasmtime` for original development
history.

This patch also contains code written by Joey Gouly
<joey.gouly@arm.com> and contributed to the above branch. These
contributions are "Copyright (c) 2020, Arm Limited."

Co-authored-by: Julian Seward <jseward@acm.org>
Co-authored-by: Benjamin Bouvier <public@benj.me>
Co-authored-by: Joey Gouly <joey.gouly@arm.com>

cranelift/codegen/src/isa/arm64/mod.rs

@@ -1,2 +1,6 @@
 mod abi;
 mod inst;
+mod lower;
+
+/// Placeholder for later implementation.
+pub struct Arm64Backend {}