Add validation for C language symbol name

## current
```bash
$ echo 'p 0' > test.rb
$ mrbc -B1234 test.rb
$ cat test.c
#include <stdint.h>
const uint8_t 1234[] = {
0x52,0x49,0x54,0x45,0x30,0x30,0x30,0x31,0x9a,0x00,0x00,0x00,0x00,0x56,0x4d,0x41,
0x54,0x5a,0x30,0x30,0x30,0x30,0x49,0x52,0x45,0x50,0x00,0x00,0x00,0x38,0x30,0x30,
0x30,0x30,0x00,0x01,0x00,0x00,0x00,0x00,0x00,0x28,0x00,0x01,0x00,0x03,0x00,0x00,
0x00,0x04,0x00,0x80,0x00,0x06,0x01,0x3f,0xff,0x83,0x00,0x80,0x00,0xa0,0x00,0x00,
0x00,0x4a,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x01,0x00,0x01,0x70,0x00,0x45,0x4e,
0x44,0x00,0x00,0x00,0x00,0x08,
};
```

## fixed
```bash
$ mrbc -B1234 test.rb
1234: Invalid C language symbol name
```