Verify signature of start lang item

[bjorn3]

In https://github.com/bjorn3/rustc_codegen_cranelift/issues/1329 it was found if the signature of the start lang item is wrong, rustc with the LLVM backend will silently compile it despite the ABI mismatch. Cg_clif crashes entirely. To avoid both issues rustc should validate the signature.

[asquared31415]

@rustbot claim

I have attempted to do this in the past (and never ended up finishing it), but I was not aware of the mentioned sigpipe: u8 argument. Where would I find information on that?

[bjorn3]

#97802 is the PR that added it.