Rollup merge of #85275 - CDirkx:memchr, r=m-ou-se

Move `std::memchr` to `sys_common`

`std::memchr` is a thin abstraction over the different `memchr` implementations in `sys`, along with documentation and tests. The module is only used internally by `std`, nothing is exported externally. Code like this is exactly what the `sys_common` module is for, so this PR moves it there.

library/std/src/lib.rs

@@ -530,7 +530,6 @@ mod sys;
 pub mod alloc;
 
 // Private support modules
-mod memchr;
 mod panicking;
 
 // The runtime entry point and a few unstable public functions used by the