elf: force filling the padding with zero bytes, add -orig-ids

See <armijnhemel/binaryanalysis-ng@1b88f40#diff-3a730cd59a811844bc0277c82828361ffb0b452f5739135300947f8948ae1f10>

This is actually also written in the spec, see
<https://man7.org/linux/man-pages/man5/elf.5.html>:

```
EI_PAD Start of padding.  These bytes are reserved and set
       to zero. [...]
```

Author: generalmimon

executable/elf.ksy

@@ -57,12 +57,14 @@ seq:
       Specifies which OS- and ABI-related extensions will be used
       in this ELF file.
   - id: abi_version
+    -orig-id: e_ident[EI_ABIVERSION]
     type: u1
     doc: |
       Version of ABI targeted by this ELF file. Interpretation
       depends on `abi` attribute.
   - id: pad
-    size: 7
+    -orig-id: e_ident[EI_PAD]..e_ident[EI_NIDENT - 1]
+    contents: [0, 0, 0, 0, 0, 0, 0]
   - id: header
     type: endian_elf
 instances: