add freebsd to KERNELnames

since we have freebsd in PERL.DISTROnames (at least for moar)
and in KERNEL.name (for all backends)

Without the change in src/core/Perl.pm test 29 in t/spec/S02-magicals/KERNEL.t
failed with perl6-m on FreeBSD (10.0-RELEASE-p9). After the change spectest-m
is clean.

Regarding the comment in src/core/Kernel.pm: If a new $*DISTRO.name is added
(as in commit 52b1dc3430025cda6b7d0617647af58c740995ef) KERNELnames
should probably be modified as well.