1.0.4.16: some GC and interrupt refactoring

1.0.4.16: some GC and interrupt refactoring

 * Conditionalize foreign_function_call_active on
   FOREIGN_FUNCTION_CALL_FLAG for clarity, and excice it from x86
   assember code.

 * Rename interrupt_maybe_gc to cheyneygc_handle_wp_violation, and
   move it cheyneygc.c. Don't pretend it is a signal handler, but pass
   in just the context and faulting address it needs.

 * Make gc_trigger_hit to accept the faulting address directly, since
   we already have it handy, and move it to cheneygc.c.

 * Rename interrupt_maybe_gc_int to maybe_gc, and move it to
   gc-common.c. Don't pretend it is a signal handler, but pass in just
   the context it needs.

 * maybe_gc_pending unused, deleted.

 * Only a single bytes_consed_between_gcs declaration, in gc.h.

 * Whitespace.

 No significant functional changes. Tested on x86/GENCGC, PPC/GENCGC,
 and PPC/Cheney.

 ...did you know that stepping is broken on PPC/Cheney but not on
 PPC/GENCGC?