Remove `cranelift-codegen`'s `build.rs`

[fitzgen]

We should check in all generated code, since it doesn't change based on any user of cranelift-codegen's input or system or anything like that, and have a script that we can re-run as needed to regenerate these files. This will speed up compile times and remove cargo build time dependencies.

See also this thread #3506 (comment)

[cfallin]

As noted in #3540, possibly a good way to go about this would be to maintain a small build.rs that just does a check for stale sources by comparing the meta-sources against a manifest, as #3534 does for ISLE. I think we should try to protect against this otherwise somewhat probable silent failure for new contributors; hopefully a build.rs that just uses sha2 and nothing else is relatively fast to compile :-)

[alexcrichton]

Given the experience from #4066 it seems we're pretty unlikely to reverse direction at this point, so I'm going to close.