Followed up with fb6f2550280c62fa3ac74f9abad4398529d5f0e2

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.

In the past, bundler updated platform without normalized variable like arm64-darwin-23.
We ignored that update. But the current bundler uses arm64-darwin for that.

* YJIT: print msg to stderr when RubyVM::YJIT.disasm not available

Print a more useful error message when people try to use this
feature without YJIT dev.

Also fix an issue with .gitignore file on macOS

* Update yjit.rb

Co-authored-by: Randy Stauner <randy@r4s6.net>

* Use warn and always return nil if YJIT disasm not available.

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Co-authored-by: Randy Stauner <randy@r4s6.net>