runs better.  Currently
     the powerpc32 is used, but only because it's the default for
     <code>powerpc*</code>.
<li> PPC630: Rewrite <code>mpn_addmul_1</code>, <code>mpn_submul_1</code>, and
     <code>mpn_mul_1</code>.  Use both integer and floating-point operations,
     possibly two floating-point and one integer limb per loop.  Split operands
     into four 16-bit chunks for fast fp operations.  Should easily reach 9
     cycles/limb (using one int + one fp), but perhaps even 7 cycles/limb
     (u