Conditionally move ppc closure_tramp and undefined_tramp to read-only space.
* This is more a proof-of-concept than anything that should be
committed upstream without careful thought.
* Create the two new assembly-routines for the trampolines.
* When the :READ-ONLY-TRAMPS build feature is enabled, use
assembly-routine references instead of foreign references for the
trampolines... And disable building the runtime version of the
trampolines.
* This causes no new failures in the test suite, which seems odd
because it almost certainly breaks SOMETHING in the debugger,
probably to do with SB!KERNEL:MAKE-LISP-OBJ.