Follow-up to MDEV-12534: Align srv_sys

Allocate srv_sys statically so that the desired alignment can be
guaranteed. This silences -fsanitize=undefined warnings.
There probably is no performance impact of this, because the
reason for the alignment to ensure the absence of false sharing
between counters. Even with the misalignment, each counter would
have been been aligned at 64 bits, and the counters would reside
in separate cache lines.