Switch enable-multilib to default

[Chris Johns]

Ralf Corsepius wrote:
> 
> The purpose of this step is to encourage people to build RTEMS
> multilib'ed, to extend exposure of multilibs and to shake out
> multilib-related issues early in RTEMS development process.
> (ATM, we have a pretty poor footprint on such issues)
> 

I think this change is a good idea although it does increase build time.

I am concerned about the current state of the m68k target given it will not 
build with the --enable-multilib option due to GCC PR32307. Does this mean we 
will not be able to ship a 4.9 release until it is fixed ?

Regards
Chris