Add WITH_REPRODUCIBLE_BUILD src.conf(5) knob

MFC r310128: 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.

MFC r310268: Build loaders reproducibly when WITH_REPRODUCIBLE_BUILD

When WITH_REPRODUCIBLE_BUILD=yes is set in src.conf(5), eliminate the
time, user, and host from the loader's version information. This allows
builds to produce bit-for-bit identical output.