libgcc move to the top level

[Paolo Bonzini]

>> If possible, I would like a proof of concept that it is possible to 
>> migrate the building of the extra parts incrementally without breaking 
>> non-converted targets, before committing (unless another maintainer 
>> overrules me).
> 
> OK, I'll do that (later).

Just to make it clearer, I'm ok with having to convert a directory at a 
time (arm, rs6000, i386, ...).  So, if your proof-of-concept works only 
with one target, but breaks only those targets that share the same cpu 
directory, I'm fine with this.

Paolo