Fix Linux crash

If config'd without -d (--debug), asynctest was crashing with:
*** longjmp causes uninitialized stack frame ***

This is because gcc will add certain checks for some functions
(including longjmp). The checks assume you can only longjmp down the
stack not up. However, if we are actually jumping to a different
fibre then it can appear as if we are going up the stack when we are
not really. This change disables the check.

Reviewed-by: Rich Salz <rsalz@openssl.org>
