[arm] Rework multlib builds for symbianelf

Symbianelf used to build multilib for armv5t with softfp, but that
architecture doesn't really support floating point instructions.  This
patch reworks the multilib configuration to use armv5te as the base
when building for floating point.

I'm not sure just how useful the symbian port is these days, so this
has only been very lightly tested (checks that libgcc builds for all
multilib variants).  Perhaps we should consider deprecating this
config?

	* config/arm/t-symbian: Rewrite for new option infrastructure.

From-SVN: r249304