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.

library/std/src/sys_common/mod.rs

@@ -24,7 +24,6 @@ pub mod backtrace;
 pub mod fs;
 pub mod io;
 pub mod lazy_box;
-pub mod memchr;
 pub mod once;
 pub mod process;
 pub mod thread;