* Crypto: Don't use "short exponent" on faster platforms.
   Rebuild router identity if key length doesn't match setting.
   This affects RI and LS encryption keys, and DH and YK.
   Faster = 64 bit with working jbigi, non-ARM, non-GNU (for now)
   Override default with crypto.elGamal.useLongKey=true/false
   LS key is built at client startup (Dest key unused)
   This will churn most of the RIs at the release.
 * SystemVersion: Add isARM()