[RFC v2] fix regressions with target-async

[Tom Tromey]

Pedro> But this looks bogus to me -- it's only reachable if you already
Pedro> have the target pushed because you're debugging a threaded core,
Pedro> and then do "attach"...

FWIW, gdb doesn't recognize multi-threaded core files on the AIX machine
to which I have access.  It seems that perhaps it is intended to work,
but neither my branch nor the gdb 7.6 installed on that machine do in
fact work.

Tom