register corruption under MS Windows / x86-64

[Torbjörn Granlund]

  It is a flaw in our testing setup that this calling convention breach is
  not caught by the automated testing.  I will fix both bugs.  :-)
  
I have now pushed a fix for the mainline repo.  Testing would be
appreciated!

Improving test suite calling conventions checks (through
tests/x86call.asm) still needs to be done.

  Unfortunately the same issue might affect released versions of GMP.  It
  is not immediately clear, as all problem code lives in x86_64/fastsse,
  code which is then explicitly included from CPU specific subdirs.

This is still pending.

-- 
Torbjörn
Please encrypt, key id 0xC8601622