kldxref: Prefer sys/cdefs.h __unreachable over the builtin

The builtin_unreachable macro provided by Clang and GCC is a hint to
the compiler used for optimization. The programs work fine even if the
compiler doesn't support it. The sys/cdefs.h has had unreachable for
9 years (commit 732b31de5d9244bd1cc98192e09ee1881e9f55e9). It expands
to the builtin if it is available. In the rare case that it is
unsupported it expands to a null statement so compilation does not
fail.

Signed-off-by: Collin Funk <collin.funk1@gmail.com>
Reviewed by: imp, freebsd@igalic.co
Pull Request: https://github.com/freebsd/freebsd-src/pull/1117