0.8.3.76

0.8.3.76
     GC fixes and miscellaneous commentary:

     Frob VOP attributes for {push-words-on,pop-words-from}-c-stack,
     per APD comment

     Comment: why WITH-PINNED-OBJECTS doesn't need an UNWIND-PROTECT

     In x86 pseudo-atomic start, clear interrupted bit _before_
     setting atomic bit, to eliminate window where a pa section might
     be interrupted immediately after starting then have the evidence
     of interruption erased

     When scavenging thread control stacks on gencgc, also look at live
     interrupt contexts that might not be on the current stack (e.g.
     alternate signal stacks).

     When stopping a thread so it may be GCed, save the signal context
     _before_ decrementing countdown_to_gc

     In lose(), send SIGSTOP to all threads other than the currently
     losing one, so we have a chance to examine what went wrong before
     they stomp all over memory