Add basic getcpu(2) support to linuxulator. 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.

This unbreaks Java from CentOS 8
(java-11-openjdk-11.0.5.10-0.el8_0.x86_64).

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22972