Kernel+LibC: Fix various build issues introduced by ssize_t

Now that ssize_t is derived from size_t, we have to

Libraries/LibC/stdio.cpp

@@ -993,7 +993,7 @@ void dbgputch(char ch)
     syscall(SC_dbgputch, ch);
 }
 
-int dbgputstr(const char* characters, int length)
+ssize_t dbgputstr(const char* characters, ssize_t length)
 {
     int rc = syscall(SC_dbgputstr, characters, length);
     __RETURN_WITH_ERRNO(rc, rc, -1);