Fix FreeBSD crash after r233837

The FreeBSD debug register access is a little usual, but in any case
different from Linux. As it stands it's not possible to share an
implementation of DR_OFFSET, so revert that part of r233837 and provide
a separate FreeBSD and Linux implementation.

We'll still want a better fix, but this should restore basic
functionality (and the buildbot).

llvm-svn: 234048