Better multilib defaults

The current multilib options for ARM are ancient and don't reflect the current architectures, instruction sets, or floating point options. Update.

Terry from ARM posted similar:
 http://gcc.gnu.org/ml/gcc-patches/2011-09/msg00683.html

which is in the ARM R&M embedded branch. The thread died.

This patch which unifies the floating point ABI flags may help:
 http://gcc.gnu.org/ml/gcc-patches/2011-03/msg00045.html

Not really on mission, but Michael is keen.