There doesn't seem to be much sense in defaulting "on" unwind tables on amd64
and not on other arch. It causes surprising differences between platforms,
such as the PR below.
Prior to this change, FreeBSD inherited the default implementation of the
method from the Gnu.h Generic_Elf => Generic_GCC parent class, which returned
true only for amd64 targets. Override that and opt on always, similar to,