Sandybridge addmul_N challenge

[Torbjorn Granlund]

I just realised a platform where your karatsuba addmul_2 could help a
lot: atom-64.  There, addmul_2 currently runs at 20 c/l.  The pipeline
is two-way superscalar and in-order.  The mul insn can be repeated
someting like one every 16th cycle.

There is a dual-core atom-64 in the test systems setup, hehe.gmplib.org.
It is currently offline for various reasons.

-- 
Torbjörn