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.

Reviewed by: bapt
MFC after: 1 month
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8842