new: dev: Add aligned memory allocation support to isc_mem

Restores the flags-based isc_mem_*x() API with ISC_MEM_ALIGN()
(removed earlier as unused) so that the upcoming false-sharing
fixes for the zone and database structures can request
cache-line-aligned memory, honored on every allocator path
including non-jemalloc builds.

Merge branch 'ondrej/re-add-aligned-allocations' into 'main'

See merge request isc-projects/bind9!12512