JENT: streamline GCD code

To avoid adding unnecessary cache lines into the _init code which would
alter the entropy rate to be analyzed, the insertion of a measurement
into the GCD array is implemented with an inline function now.

Further, the change fixes an off-by one in the GCD analyzer code.

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