Enable reproducible builds in advance of 12.0-REL

r338642 toggled the REPRODUCIBLE_BUILD knob but missed the
corresponding kern.opts.mk change.

We want to build the 12.0 release artifacts with reproducible builds
mode enabled. Switch it on in HEAD now to enable testing with upcoming
ALPHA builds. We can revisit the default setting for HEAD after the
branch is created.

This change eliminates the build metadata (user, hostname, timestamp,
etc.) from the kernel and loader. If the src tree is a git, svn or p4
checkout with changes then the metadata is retained.

The WITHOUT_REPRODUCIBLE_BUILD src.conf(5) knob can be used to revert
to the previous behaviour.

Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation