Bug 5008: SIGBUS in PagePool::level() with custom rock slot size (#515)

Bug 5008: SIGBUS in PagePool::level() with custom rock slot size (#515)

SMP Squids were crashing on arm64 due to incorrect memory alignment of
Ipc::Mem::PagePool::theLevels array. The relative position of the array
depends on the number of workers and the number of pages (influenced by
the cache capacity and slot size), so some configurations worked OK.

We have to manually align manually positioned fields inside shared
memory segments. Thankfully, C++11 provides alignment-computing APIs.