a 64-bit inverse and take
     64-bits at a time from the dividend, as per the 32-bit divisor case in
     mpn/sparc64/mode1o.c.  This must be done in assembler, since the full
     64-bit registers (<code>%gN</code>) are not available from C.
<li> UltraSPARC/32: <code>mpn_divexact_by3c</code> can work 64-bits at a time
     using <code>mulx</code>, in assembler.  This would be the same as for
     sparc64.
<li> UltraSPARC: <code>modlimb_invert</code> might save a few cycles from
     masking down to just t