Cygwin: x86_64: pthreads: Install exception handler after switching stack

Cygwin: x86_64: pthreads: Install exception handler after switching stack

After creating a pthread, the stack gets moved to the desired memory
location.  While the 32 bit thread wrapper copies the exception handler
information to the new stack (so we have at least *some* exception
handler present), the x86_64 code didn't perform any exception handler
magic.  Fix that.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>