New testcase gdb.threads/leader-exit-attach.exp (PR threads/8153)

Add a new testcase for exercising attaching to a process after its
main thread has exited.

This is not possible on Linux, the kernel does not allow attaching to
a zombie task, so the test is kfailed there.  It is possible however
on Windows at least, and was the scenario addressed by the Windows
backend fix in
https://sourceware.org/legacy-ml/gdb-patches/2003-12/msg00479.html,
nowadays PR threads/8153, back in 2003.

Passes cleanly on Cygwin.
KFAILed on GNU/Linux native and gdbserver.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=8153
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31554
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31555
Change-Id: Ib554f92f68c965bb4603cdf2aadb55ca45ded53b