Expose the kernel's build-ID through sysctl
After our migration (of certain architectures) to lld the kernel is built
with a unique build-ID. Make it available via a sysctl and uname(1) to
allow the user to identify their running kernel.
Submitted by: Ali Mashtizadeh <ali_mashtizadeh.com>
MFC after: 2 weeks
Event: Waterloo Hackathon 2019
Differential Revision: https://reviews.freebsd.org/D20326