Rollup merge of #147015 - Zalathar:dispose-tm, r=lqd

Use `LLVMDisposeTargetMachine`

After bumping the minimum LLVM version to 20 (rust-lang/rust#145071), we no longer need to run any custom code when disposing of a TargetMachine, so we can just use the upstream LLVM-C function.

Author: Zalathar