Auto merge of #31057 - bluss:memrchr-fallback, r=alexcrichton
Auto merge of #31057 - bluss:memrchr-fallback, r=alexcrichton

Use the fallback impl for memrchr on non-linux

The memrchr code was never used(!). This brings the memrchr improvements to
non-linux platforms (LineWriter / buffered stdout benefits).

Previous PR #30381

GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust