Enabling the WITH_REPRODUCIBLE_BUILD knob for 12.0-REL

The FreeBSD base system is a reproducible build[1] with a minor
exception: the build metadata (timestamps, user, hostname, etc.)
included in the kernel and loader.

With the default, non-reproducible build the kernel ident looks like:

FreeBSD 12.0-ALPHA5 #4 r338195: Mon Jan 1 10:11:12 EDT 2018
   user_at_hostname:/path/to/freebsd/src

and the loader ident:

FreeBSD/amd64 EFI loader, Revision 1.1
(Mon Jan 1 10:11:12 EDT 2018 user_at_hostname)

With reproducible builds enabled the kernel ident looks like:

FreeBSD 12.0-ALPHA5  r338195

and the loader ident:

FreeBSD/amd64 EFI loader, Revision 1.1

I would like to enable the REPRODUCIBLE_BUILD knob by default for the
12.0 release, and propose we do this by adding a step to switch the
default to the list of changes[2] that re_at_ commits to the branch as
part of the release process.

[1] https://reproducible-builds.org
[2] https://www.freebsd.org/doc/en_US.ISO8859-1/articles/freebsd-releng/releng-head.html