Please update addaddmul_1msb0.asm to support ABI in mingw64

[Torbjörn Granlund]

nisse at lysator.liu.se (Niels Möller) writes:

  L(top):
  	mov	(ap, n, 8), %rdx
  	mulx 	%r8, alo, hi
  	adox	ahi, alo
  	mov	hi, ahi			C 2-way unroll.
  	adox	zero, ahi		C Clears O
  	
  	mov	(bp, n), %rdx
  	mulx 	%r9, blo, hi
  	adox	bhi, blo
  	mov	hi, bhi
  	adox	zero, bhi		C clears O

  	adc	blo, alo		C Or sbb, for addsubmul_1msb0
  	mov	alo, (rp, n, 8)
  	inc	n
  	jnz	top

Problem: the adc will write a useless value to the O flag.  That is then
read by the first adox, yielding incorrect results.  Clearing O without
creating any (too bad false) dependencies could perhaps be done with an
additional dummy adox zero, zero.

Another remedy would be to use adcx instead of adc, but then we cannot
easily make a addsubmul_1msb0 variant.

-- 
Torbjörn
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