The -Wbuiltin-declaration-mismatch warning below should probably only be issued if -Wimplicit-function-declaration is disabled, otherwise it's mostly redundant (although only one instance of the latter warning is issued for each implicit declaration regardless of the number of calls).

In addition, the <built-in>: note  after the -Wbuiltin-declaration-mismatch warning is meaningless.

$ cat t.c && gcc -O2 -S -Wall -Wextra t.c
int f (const char *s)
{
  return memcmp (s, "1234", 4);
}
t.c: In function ‘f’:
t.c:3:10: warning: implicit declaration of function ‘memcmp’ [-Wimplicit-function-declaration]
    3 |   return memcmp (s, "1234", 4);
      |          ^~~~~~
t.c:3:29: warning: ‘memcmp’ argument 3 type is ‘int’ where ‘long unsigned int’ is expected in a call to built-in function declared without prototype [-Wbuiltin-declaration-mismatch]
    3 |   return memcmp (s, "1234", 4);
      |                             ^
<built-in>: note: built-in ‘memcmp’ declared here