Add WITH_REPRODUCIBLE_BUILD src.conf(5) knob to disable kernel metadata

The kernel builds reproducibly, except for the time, date, user, and
hostname baked into the kernel (reported at startup and via the
kern.version sysctl for uname). Add a build knob to disable the
inclusion of this metadata.

Reviewed by: jhb
MFC after: 1 month
Relnotes: Yes
Sponsored by: Reproducible Builds World Summit 2, Berlin
Differential Revision: https://reviews.freebsd.org/D8809