The default definition of setjmp which is implemented in MinGW-w64
cannot be used with programs like QEMU which call longjmp from
code without structured exception handling (SEH).

This code therefore disables stack unwinding.

We could also implement SEH for QEMU's generated JIT code, but
that is much more difficult. Stack unwinding would also cost
execution time.

Signed-off-by: Stefan Weil <sw@weilnetz.de>