[tree-ssa] Mainline merge plan

[Richard Earnshaw]

> 
> The SC approved the plan to merge tree-ssa into mainline.  Tomorrow
> Feb/25, the branch will be closed to new features that do not advance
> items in this plan or fix an existing PR.

I hope the GMP dependencies I reported in an earlier message are also 
going to be sorted out before the merge.

I'd also like to see arm-elf building as well (you have no ARM target in 
your list).

R.