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.

Author: krtab

library/std/src/io/buffered/linewritershim.rs

@@ -1,5 +1,5 @@
 use crate::io::{self, BufWriter, IoSlice, Write};
-use crate::sys_common::memchr;
+use core::slice::memchr;
 
 /// Private helper struct for implementing the line-buffered writing logic.
 /// This shim temporarily wraps a BufWriter, and uses its internals to

library/std/src/io/mod.rs

@@ -304,7 +304,7 @@ use crate::ops::{Deref, DerefMut};
 use crate::slice;
 use crate::str;
 use crate::sys;
-use crate::sys_common::memchr;
+use core::slice::memchr;
 
 #[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
 pub use self::buffered::WriterPanicked;

library/std/src/sys/hermit/memchr.rs

@@ -9,7 +9,6 @@ use crate::path::{self, PathBuf};
 use crate::str;
 use crate::sync::Mutex;
 use crate::sys::hermit::abi;
-use crate::sys::memchr;
 use crate::sys::unsupported;
 use crate::vec;
 

library/std/src/sys/hermit/os.rs

@@ -21,7 +21,6 @@ pub mod fd;
 pub mod fs;
 #[path = "../unsupported/io.rs"]
 pub mod io;
-pub mod memchr;
 pub mod net;
 pub mod os;
 #[path = "../unix/os_str.rs"]

library/std/src/sys/sgx/memchr.rs

@@ -40,7 +40,6 @@ pub mod pipe;
 pub mod process;
 pub mod stdio;
 pub use self::itron::thread;
-pub mod memchr;
 pub mod thread_local_dtor;
 pub mod thread_local_key;
 pub use self::itron::thread_parking;

library/std/src/sys/sgx/mod.rs

@@ -23,7 +23,6 @@ pub mod kernel_copy;
 #[cfg(target_os = "l4re")]
 mod l4re;
 pub mod locks;
-pub mod memchr;
 #[cfg(not(target_os = "l4re"))]
 pub mod net;
 #[cfg(target_os = "l4re")]