Speed up char-downcase, char-upcase and char-equal.

By using a flat vector for case information instead of a vector with
conses.
Fix do_pending_interrupt on ppc.

twllei reg_ZERO, trap_PendingInterrupt
never worked because reg_ZERO is a lisp-side invention.
Use an unconditional trap instead.
Fix maybe_gc on precises GCs.

Do not call fake_foreign_function_call if it's already in C.
That makes heap_exhausted_error work on ARM.