- Enable native compilation for arm.

- to do this, and make the result runnable on pre-thumb arm, change
  upstreams assembler snippets and stubs to use "mov pc,reg" instead
  of the return/call-to-thumb-friendly "bx reg", which is not available
  in non-thumb-enabled arm CPUs. Whether this is the way to go, or a
  seperate "armt" architecture for ocaml is needed, needs to be discussed
  with upstream.

Resulting compiler, when running its selftest suite, has 6 errors less than
the same on i386. Unison compiled natively with this passes its self-test.

(is)