Make stack 16-byte aligned.

lisp/x86-assem.S:
o Make sure the stack is 16-byte aligned in the alloc_overflow_foo and
  alloc_to_foo routines.  These eventually call into C code, and the
  stack is required to be 16-byte aligned on Darwin.  We apply this to
  all x86 implementations since it's harmless.
o Did not update the alloc_8/16_to_foo routines because they are going
  to be deleted.

x86/macros.lisp:
o Don't call the alloc_8/16_to_foo routines when we're not doing
  inline allocation.  I don't think there's much to be gained with
  these special functions and maintainence is a pain with assembly
  code.