ARM: default to arm1176jzf-s for hard-float platforms.

It makes no sense to try and compile for arm7tdmi when we're targeting
something like gnueabihf. Although not strictly the most basic hardware
conceivable, I believe arm1176jzf-s is a reasonable compromise (that can always
be overridden explicitly if needed) since it's still in reasonably common use
unlike earlier cores.

Patch by Stephen Kelly.