Compile Jitter RNG without optimizations

The Jitter RNG takes its entropy from differences in the execution time
of the SHA3 operation. To avoid potential shortcuts introduced by the
compiler, the code is compiled without optimizations.

Without optimizations, sufficient entropy is considered to be harvested
on different CPU architectures.

Suggested-by: Kurt Roeckx
Suggested-by: Joshua E. Hill
Signed-off-by: Stephan Mueller <smueller@chronox.de>