ARM XOR encoder not decoding the whole payload

After manually applying #1923 I noticed that the arm xor encoder is broken for longer payloads.

https://github.com/Gallopsled/pwntools/blob/bd12d1874f17e1fd6a9b26411ccc7ccd6c31f4cb/pwnlib/encoders/arm/xor.py#L29-L46

`maximum` is hardcoded to `256`, so it only decodes `maximum - length` bytes instead of all of `length`.
My quick fix was to set `maximum = len(raw_bytes)*2`, but I'm not sure if that violates some guarantees for the blacklist.

Test:
```python
from pwn import *
context.arch = 'arm'

arm_shellcode = asm(shellcraft.execve('/bin/sh', ['/bin/sh', '-c', 'ls -la'], 0))
arm_shellcode = encoder.encode(arm_shellcode, avoid=b'\x00\n')

exe = ELF.from_bytes(arm_shellcode)
io = process(['/usr/bin/qemu-arm', exe.path])
io.interactive()
```

Start in pwnlib/encoders/arm/xor.py at the linked lines and reproduce the issue with the ARM shellcode and qemu-arm command from the report. Check that longer payloads decode completely while the requested blacklist remains valid; the issue is done when the provided execution test succeeds without truncation.

Written by the indexing model from the issue text.