gdb.threads/thread-execl, don't re-exec forever

I noticed on Cygwin, gdb.thread/thread-execl.exp would hang, (not that
surprising since we can't follow-exec on Cygwin).  Looking at the
process list running on the machine, we end up with a thread-execl.exe
process constantly respawning another process [1].

We see the same constant-reexec if we launch gdb.thread/thread-execl
manually on the shell:

 $ ./testsuite/outputs/gdb.threads/thread-execl/thread-execl
 # * doesn't exit, constantly re-execing *
 ^C

Prevent this leftover constantly-re-execing scenario by making the
testcase program only exec once.  We now get:

  $ ./testsuite/outputs/gdb.threads/thread-execl/thread-execl
  $   # exits immediately after one exec.

On Cygwin, the testcase now fails reasonably quickly, and doesn't
leave stale processes behind.

Still passes cleanly on x86-64 GNU/Linux.

[1] Cygwin's exec emulation spawns a new Windows process for the new
image.

Approved-By: Andrew Burgess <aburgess@redhat.com>
Change-Id: I0de1136cf2ef7e89465189bc43489a2139a80efb