PowerPC:
* create and use trampolines for long calls

X86:

SPARC:
* The inc_stack code saves more argument registers than
  necessary: a C callee won't clobber %l or %i regs.
* Does noproc_primop_interface_N really need to save and
  restore FCALLS/HP/RA/NSP around P-less primop calls?
  (x86 doesn't save and restore HP in this interface.)
  Ditto for nocons_nofail_primop_interface_0.
