+377: Memory fault error reporting
+ On those architectures where :C-STACK-IS-CONTROL-STACK is in
+ *FEATURES*, we handle SIG_MEMORY_FAULT (SEGV or BUS) on an altstack,
+ so we cannot handle the signal directly (as in interrupt_handle_now())
+ in the case when the signal comes from some external agent (the user
+ using kill(1), or a fault in some foreign code, for instance). As
+ of sbcl-0.8.20.20, this is fixed by calling
+ arrange_return_to_lisp_function() to a new error-signalling
+ function, but as a result the error reporting is poor: we cannot
+ even tell the user at which address the fault occurred. We should
+ arrange such that arguments can be passed to the function called from
+ arrange_return_to_lisp_function(), but this looked hard to do in
+ general without suffering from memory leaks.
+
+378: floating-point exceptions not signalled on x86-64
+ Floating point traps are currently not enabled on the x86-64 port.
+ This is true for at least overflow detection (as tested in
+ float.pure.lisp) and divide-by-zero.
+
+379: TRACE :ENCAPSULATE NIL broken on ppc/darwin
+ See commented-out test-case in debug.impure.lisp.