Support for w*ndows

[Torbjorn Granlund]

Torbjorn Granlund <tg at gmplib.org> writes:

  I did something similar to the mechanism for nails.  An assembly file
  that supports more than the standard ABI need to provide ABI
  declarations:
  
  ABI_SUPPORT(DOS64)
  ABI_SUPPORT(ELF64)
  
  In configure, for environments that do not comply to the standard ABI,
  we need to issue a statement declaration
  
    AC_SUBST(GMP_NONSTD_ABI,DOS64)
  
  Finally, in the loop for selecting mpn code, when GMP_NONSTD_ABI is set,
  files without the corresponding ABI_SUPPORT seclaration will be skipped.
  
  Things seem to work well on a virtual dos64 machine around here.

I pushed this change.

I have converted 4 assembly files with 5 functions to support Windoze-64
under cygwin and mingw64, providing mul_1, addmul_1, submul_1,
mul_basecase, and sqr_basecase.

For the first 3 functions I made the port in a tricker way, resulting in
no extra overhead.  The last 2 files just use a mechanism similar to the
one Niels described which is used by Nettle crypto library.

It probably could take some cleanup; some things are now repeated for
each asm file.  The DOS64_ENTRY macro is somewhat dumb, and cannot
handle sign/zero extension of stack-based parameters.  It therefore only
handles the (up to) 4 register parameters.

Converting more asm files is not hard, but it is also no terrible hurry.
The improved configure will use the converted files and ignore the rest.

-- 
Torbjörn