Use now available link name "signal" instead of "bsd_signal"
On android, the `bsd_signal` is gone, the `signal` is available.
While this is the most obvious solution, I am not sure of a few things:
- How are we going to keep compatibility with older NDKs where `signal` does not exist;
- Was something dependent on this being different on android and thus would break (for example, the rust compiler uses this function, so it may break somewhere).
Fixes#236.
60f90cf560