vm_dump: FreeBSD ports' libexecinfo's backtrace(3) can't trace
beyond signal trampoline, and as described in r38342 it can't
trace on -O because it see stack frame pointers.
libunwind unw_backtrace see dwarf information in the binary
and it works with -O (without frame pointers).
configure.in: remove r38342's hack and check libunwind.