adding POSIX memccpy and mempcpy GNU extension.

[memccpy](https://pubs.opengroup.org/onlinepubs/9699919799/functions/memccpy.html)

[mempcpy](https://man7.org/linux/man-pages/man3/mempcpy.3.html)

(backport <#4186>)
(cherry picked from commit ba5930d)

libc-test/semver/linux-gnu.txt

@@ -650,6 +650,7 @@ malloc_stats
 malloc_trim
 malloc_usable_size
 mallopt
+mempcpy
 mq_notify
 nl_mmap_hdr
 nl_mmap_req

libc-test/semver/unix.txt

@@ -638,6 +638,7 @@ localtime_r
 lseek
 lstat
 malloc
+memccpy
 memchr
 memcmp
 memcpy

src/unix/linux_like/linux/gnu/mod.rs

@@ -1539,6 +1539,8 @@ extern "C" {
         timeout: *const crate::timespec,
         sigmask: *const crate::sigset_t,
     ) -> c_int;
+
+    pub fn mempcpy(dest: *mut c_void, src: *const c_void, n: size_t) -> *mut c_void;
 }
 
 cfg_if! {

src/unix/mod.rs

@@ -681,6 +681,7 @@ extern "C" {
     pub fn memcpy(dest: *mut c_void, src: *const c_void, n: size_t) -> *mut c_void;
     pub fn memmove(dest: *mut c_void, src: *const c_void, n: size_t) -> *mut c_void;
     pub fn memset(dest: *mut c_void, c: c_int, n: size_t) -> *mut c_void;
+    pub fn memccpy(dest: *mut c_void, src: *const c_void, c: c_int, n: size_t) -> *mut c_void;
 }
 
 extern "C" {