shared/libc/string0: Don't include string.h, and provide __memcpy_chk.

Some toolchains will have string.h defining various macros which can lead
to compile errors for string function implementations.  Not including
string.h fixes this.

An implementation of __memcpy_chk is provided for toolchains that enable
_FORTIFY_SOURCE.

Fixes issue #6046.

Signed-off-by: Alexey 'alexxy' Shvetsov <alexxyum@gmail.com>

shared/libc/string0.c

@@ -25,7 +25,7 @@
  */
 
 #include <stdint.h>
-#include <string.h>
+#include <stddef.h>
 
 #define likely(x) __builtin_expect((x), 1)
 
@@ -64,6 +64,13 @@ void *memcpy(void *dst, const void *src, size_t n) {
     return dst;
 }
 
+void *__memcpy_chk(void *dest, const void *src, size_t len, size_t slen) {
+    if (len > slen) {
+        return NULL;
+    }
+    return memcpy(dest, src, len);
+}
+
 void *memmove(void *dest, const void *src, size_t n) {
     if (src < dest && (uint8_t*)dest < (const uint8_t*)src + n) {
         // need to copy backwards