Move elf class marker figuring to helper function, generalize a bit

Move elf class marker figuring to helper function, generalize a bit

- Determine arch-specific issues by looking at the elf header instead
  of compile-time #ifdefs, so we'll generate correct dependencies for
  non-native elf binaries too. Currently this is just alpha which despite
  being a 64bit system, never had the (64bit) markers in its dependencies.
  Of course alpha has pretty much already met its mark^H^H er maker by now,
  but doing the right thing is cheap... Also we'll need similar special
  cases sooner or later for other archs (such as x32).
(cherry picked from commit f84a71cdc786a012fc9cbe16c5236c622ea970a4)