Delete architecture-specific memchr code in std::sys Currently all architecture-specific memchr code is only used in `std::io`. Most of the actual `memchr` capacity exposed to the user through the slice API is instead implemented in `core::slice::memchr`. Hence this commit deletes `memchr` from `std::sys[_common]` and replace calls to it by calls to `core::slice::memchr` functions. This deletes `(r)memchr` from the list of symbols linked to libc. The interest of putting architecture specific code back in core is linked to the discussion to be had in #113654