Add basic getcpu(2) support to linuxulator. This unbreaks
Java from CentOS 8 (java-11-openjdk-11.0.5.10-0.el8_0.x86_64).
The purpose of this syscall is to query the CPU number and the NUMA
domain the calling thread is currently running on. The third argument
is ignored. It doesn't do anything regarding scheduling - it's literally
just a way to query the current state, without any guarantees you won't
get rescheduled an opcode later.