Hi all,

  While I was looking at the dlmmap/dlmunmap code in closures.c, I started
looking into testsuite FAILs to see if any of them might be caused by the code
I had added, and stumbled across a bigger can of worms.  The code in win32.S
doesn't take account of numerous among the FFI_TYPE_xxx return types, leading
to stack-based garbage being returned, and in one place mishandles the ABI
convention that the callee should pop the hidden pointer arg for struct return
types.

  Before this patch there were 66 FAILs on libffi HEAD (sourcware) and 20 on
libffi GCC.  After, there are only one or two remaining (depending on GCC
version), both "test for excess errors" failures related to format-string
warnings and unrelated to all this.  Tested on i686-pc-cygwin; the GCC testing
only involved bubblestrapping an existing $objdir rather than running a full
bootstrap, but that should be OK for a target lib.

libffi/ChangeLog:

	PR libffi/40807
	* src/x86/ffi.c (ffi_prep_cif_machdep): Also use sign/zero-extending
	return types for X86_WIN32.
	* src/x86/win32.S (_ffi_call_SYSV): Handle omitted return types.
	(_ffi_call_STDCALL, _ffi_closure_SYSV, _ffi_closure_raw_SYSV,
	_ffi_closure_STDCALL): Likewise.

	* src/closures.c (is_selinux_enabled): Define to const 0 for Cygwin.
	(dlmmap, dlmunmap): Also use these functions on Cygwin.

  OK for both HEADs?  And would you like me to apply the closures.c fix as a
separate commit or shall I do them all together?

    cheers,
      DaveK