cranelift-codegen: move old x86 and RISC-V backends to isa/legacy/.

These backends will be removed in the future (see
bytecodealliance/rfcs#12 and the pending #3009 in this repo).

In the meantime, to more clearly communicate that they are using
"legacy" APIs and will eventually be removed, this PR places them in an
`isa/legacy/` subdirectory. No functional changes otherwise.

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

@@ -0,0 +1,12 @@
+//! Legacy ("old-style") backends that will be removed in the future.
+
+// N.B.: the old x86-64 backend (`x86`) and the new one (`x64`) are both
+// included whenever building with x86 support. The new backend is the default,
+// but the old can be requested with `BackendVariant::Legacy`. However, if this
+// crate is built with the `old-x86-backend` feature, then the old backend is
+// default instead.
+#[cfg(feature = "x86")]
+pub(crate) mod x86;
+
+#[cfg(feature = "riscv")]
+pub(crate) mod riscv;