Fix arch usage

The following example will fail without this patch.  Specifically, `nops` is an `AssemblerBlob` and uses `target_arch` which is undefined (and likely supposed to be `blocks.arch`).

```python
from pwn import *
context('i386', 'linux')

sc = shellcode.nops(10) \
   + shellcode.echo('you win!\n') \
   + shellcode.sh() \
   + shellcode.infloop()

sc = shellcode.scramble(sc, avoid='\r\n\x00')
```