gdb-remote signal delivery test cleanup.

Learned that MacOSX only accepts signal delivery on a thread that is
already signal handling.  Reworked the test exe to cause a SIGSEGV
and recover if either nothing intercepts the SIGSEGV handler, or
if a SIGUSR1 is inserted.  The test uses the latter part to test
signal delivery on continue using the SIGUSR1.

I still don't have this working on MacOSX.  I'm seeing the
signal get delivered to a different thread than the one I'm
specifying with $Hc{thread-id} + $C{signo}, or with
$vCont;C{signo}:{thread-id};c.  I'll come back to this
after getting it working on the llgs branch on Linux x86_64.