shellcraft.mov does not support AH, BH, etc. registers

```python
from pwn import *
# context.arch = 'i386'
print shellcraft.mov('eax', 'ah')
```
Expected:
`movzx eax, ah`

Result:
```
NameError: name 'ah' is not defined
```

The error is raised when trying to eval `ah` as a system-constant value

But the real reason is `ah` is not defined in `shellcraft/registers.py` as a register in `i386`.

Issue occurs in `amd64` as well.

Start in shellcraft/registers.py and reproduce the shellcraft.mov('eax', 'ah') example from the issue for i386 and amd64. Define the missing registers so they are recognized instead of evaluated as system constants, and confirm the result produces the expected movzx instruction.

Written by the indexing model from the issue text.