do not use -m64 for arm64

Incorporates patch from https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=748708
Merge pull request #404 from satta/aarch64

do not use -m64 for arm64
make sure we only translate sequence of at least one codon length
add feature_node:appears_as_root_node, many fixes
add some helper functions to speclib
extend example spec with more detailed cases