This is so that e.g. building with `--enable-zjit=dev` will test with the
disassembly feature. It makes more sense, saves on build time and
reveals that
`backend::arm64::tests::sp_movements_are_single_instruction` was in
fact failing with the `disasm` feature.
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.