[llvm-readelf] Fix emitting incorrect number of spaces in '--hex-dump'.

This patch helps teach llvm-readelf to emit a correct number spaces when
dumping in hex format.

Before this patch, when the hex data doesn't fill the 4th column, some
spaces are missing.

```
Hex dump of section '.sec':
0x00000000 00000000 00000000 00000000 00000000 ................
0x00000010 00000000 00000000 00000000 0000 ..............
```

After this patch:

```
Hex dump of section '.sec':
0x00000000 00000000 00000000 00000000 00000000 ................
0x00000010 00000000 00000000 00000000 0000     ..............
```

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D84640