Eliminate linker warnings for syscalls

test-app/app_lpc55s69.c

@@ -105,3 +105,65 @@ void main(void)
         __asm__ volatile ("wfi");
     }
 }
+
+
+#include "sys/stat.h"
+int _getpid(void)
+{
+    return 1;
+}
+
+int _kill(int pid, int sig)
+{
+    (void)pid;
+    (void)sig;
+    return -1;
+}
+
+void _exit(int status)
+{
+    _kill(status, -1);
+    while (1) {}
+}
+
+int _read(int file, char *ptr, int len)
+{
+    (void)file;
+    (void)ptr;
+    (void)len;
+    return -1;
+}
+
+int _write(int file, char *ptr, int len)
+{
+    (void)file;
+    (void)ptr;
+    return len;
+}
+
+int _close(int file)
+{
+    (void)file;
+    return -1;
+}
+
+int _isatty(int file)
+{
+    (void)file;
+    return 1;
+}
+
+int _lseek(int file, int ptr, int dir)
+{
+    (void)file;
+    (void)ptr;
+    (void)dir;
+    return 0;
+}
+
+int _fstat(int file, struct stat *st)
+{
+    (void)file;
+    st->st_mode = S_IFCHR;
+    return 0;
+}