Per-CPU variables in X15

As I started working on a Refcache prototype (Refcache being a scalable reference counting scheme introduced in the RadixVM paper), tests run with little physical memory (currently 6 MB, could be less but Grub seems to require that much) started to fail. The introduction of the delta caches blew the size of the bss section up by about 10 MB, for only 32 processors at most. It was time to provide real support for percpu variables, at least those that are statically allocated.