vyper default functions don't respect nonreentrancy keys
### Summary
Prior to v0.3.0, `__default__()` functions did not respect the `@nonreentrancy` decorator and the lock was not emitted. This is a known bug and was already visible in the issue tracker (
https://github.com/vyperlang/vyper/issues/2455) but it is being re-issued as an advisory so that tools relying on the advisory publication list can incorporate it into their searches.
A contract search was additionally performed and no vulnerable contracts were found in production.
### PoC
```vyper
@external
@payable
@nonreentrant("default")
def __default__():
pass
```
after codegen:
```
[seq,
[if, [lt, calldatasize, 4], [goto, fallback]],
[mstore, 28, [calldataload, 0]],
[with, _func_sig, [mload, 0], seq],
[seq_unchecked,
[label, fallback],
[seq,
pass,
# Line 5
pass,
pass,
# Line 4
stop]]],
```
### Impact
No vulnerable production contracts were found. Additionally, using a lock on a `default` function is a very sparsely used pattern. As such, the impact is `low`.