Fixing getrandom()

So in Libgcrypt we have been urged to move to getrandom to avoid some issues in the early boot phase and to get better performance in almost all cases. Weakening getrandowm now is a no-go and would badly reflect on the security consciousness of _some_ kernel folks.

Using Stephan Müller's jitter based entropy generator inside the kernel is by any means the Right Thing to do - even if it is for now only a fallback. In Libgcrypt's Windows version we already use it because on Windows the JitterRNG is the only non-external-hardware RNG which has been approved by Germany's BSI for use in restricted communication at the VS-NfD level. On Linux getrandom has been evaluated as fine but nevertheless we mix some entropy from the JitterRNG into our own entropy pool. Right, we also use RDRAND in addition and that is technically okay. But because RDRAND can't be evaluated the evaluation of Libgcrypt assumes that RDRAND adds 0 bits of entropy to the pool.