1.0.5.35: stack alignment on x86/Darwin, once more

1.0.5.35: stack alignment on x86/Darwin, once more

 * Stack alignment cannot be changed after arguments have been pushed
   on stack: ALLOCATE-NUMBER-STACK-SPACE is the place to do this, and
   nowhere else.

 * Use the RESET-STACK-POINTER logic on all x86 platforms for
   simplicity.

 * Factor out the alignment logic to ALIGN-STACK-POINTER.

 * Clear DF unconditionally when calling out, which means that Solaris
   doesn't need to switch it back and forth. (Darwin, Solaris, and Win32
   all need DF cleared for call-out.)