vm-engine: remove register assignments

vm-engine: remove register assignments

* libguile/vm-engine.c: Remove the register assignments inherited from
  the 1990s.  GCC does seem to allocate reasonably on systems with
  enough registers (e.g. x86-64), and on system with too few (ia32) we
  disabled manual allocation.  Anyway this code was never tested, so
  it's better to leave the compiler to do its own thing, until proven
  otherwise.  Also in the RTL VM we don't need to allocate a register to
  the SP, because it isn't accessed as much.