clang: enable unwind tables on FreeBSD !amd64

There doesn't seem to be much sense in defaulting "on" unwind tables on
amd64 and not on other arches. It causes surprising differences between
platforms, such as the PR below[1].

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, e.g., NetBSD's driver.

[1] https://bugs.freebsd.org/241562

Patch by cem (Conrad Meyer).

Reviewed By: dim

Differential Revision: https://reviews.llvm.org/D70110