Auto merge of #44906 - dkl:main-signature, r=nagisa

Auto merge of #44906 - dkl:main-signature, r=nagisa

Fix native main() signature on 64bit

Hello,

in LLVM-IR produced by rustc on x86_64-linux-gnu, the native main() function had incorrect types for the function result and argc parameter: i64, while it should be i32 (really c_int). See also #20064, #29633.

So I've attempted a fix here. I tested it by checking the LLVM IR produced with --target x86_64-unknown-linux-gnu and i686-unknown-linux-gnu. Also I tried running the tests (`./x.py test`), however I'm getting two failures with and without the patch, which I'm guessing is unrelated.