x86_64 asm and 32bit field/scalar options should be incompatible #727

1. 
   real-or-random commented at 4:41 pm on March 24, 2020: contributor

   0> ./configure  --with-field=32bit --with-scalar=32bit --with-asm=x86_64 
   1Build Options:
   2  asm                     = x86_64
   3  bignum                  = gmp
   4  field                   = 32bit
   5  scalar                  = 32bit
   

   But this does not make sense because there is no asm for the 32bit fields and scalars.

   related: #726

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