Compiler target for freebsd10 on armv6

Pulls in some small patches to the configure system and arm backend to target armv6 softfloat.  Tested with a Raspberry PI.

I have an OPAM binary for [this system](https://github.com/andrewray/mirage-fpga/releases) as well.

Hopefully with this patch we should be able to get ocaml going with;

```
$ opam init --comp=4.01.0+armv6-freebsd
```