YJIT: ZJIT: Allow both JITs in the same build

This commit allows building YJIT and ZJIT simultaneously, a "combo
build". Previously, `./configure --enable-yjit --enable-zjit` failed. At
runtime, though, only one of the two can be enabled at a time.

Add a root Cargo workspace that contains both the yjit and zjit crate.
The common Rust build integration mechanisms are factored out into
defs/jit.mk.

Combo YJIT+ZJIT dev builds are supported; if either JIT uses
`--enable-*=dev`, both of them are built in dev mode.

The combo build requires Cargo, but building one JIT at a time with only
rustc in release build remains supported.

jit.rs

@@ -0,0 +1,4 @@
+#[cfg(feature = "yjit")]
+pub use yjit::*;
+#[cfg(feature = "zjit")]
+pub use zjit::*;