main: provide weak symbols for _close, _lseek, _read, _write

This increases the size of the binary in a stm32f072 a bit:

before:
Memory region         Used Size  Region Size  %age Used
           FLASH:       17296 B       128 KB     13.20%
             RAM:        4268 B        16 KB     26.05%

after:
Memory region         Used Size  Region Size  %age Used
           FLASH:       17312 B       128 KB     13.21%
             RAM:        4268 B        16 KB     26.05%

Closes: #164

src/main.c

@@ -51,6 +51,15 @@ static void SystemClock_Config(void);
 static USBD_GS_CAN_HandleTypeDef hGS_CAN;
 static USBD_HandleTypeDef hUSB = {0};
 
+void __weak _close(void) {
+}
+void __weak _lseek(void) {
+}
+void __weak _read(void) {
+}
+void __weak _write(void) {
+}
+
 int main(void)
 {
 	HAL_Init();