#![no_builtins] should prevent a crate from participating in LTO

[Amanieu]

This attribute is needed to prevent LLVM from optimizing a memcpy implementation into a recursive call. However this attribute is not preserved in LTO builds: the attribute only works if it is set at the top-level crate.

Since crates containing implementations of built-in functions aren't meant to be called directly, they don't benefit from LTO and should be linked as a separate object.

One workaround (using rlibc as an example) is to first compile a rlibc_inner crate as a staticlib with #![no_builtins], and then compile the rlibc crate as a rlib which links to rlibc_inner.a.

cc #31544 @alexcrichton @japaric

[japaric]

OK, I looked at the code around LTO and I think I know how to implement this. Will implement it and report back.

[japaric]

It works! I'll send a PR.