Auto merge of #141415 - est31:let_chains_libcore, r=tgross35

Remove #![feature(let_chains)] from libcore

PR rust-lang/rust#132833 has stabilized the let_chains feature. This PR removes the last occurrence from the library.

Split out of #140966 as it caused breakage. Now we have a patch to the linux tree that fixes the build, so we update the linux tree to it.

cc Rust-for-Linux/linux#1163
cc rust-lang/rust#140722

Author: bors