coroutine-sigaltstack: Change jmp_buf to sigjmp_buf

This is a simple patch to change the type of old_env from jmp_buf
to sigjmp_buf.  old_env is used by sigsetjmp and as such should be
a sigjmp_buf.

This fixes a stack_chk fail in a OSX 32bit build. Since at least on
OSX sigjmp_buf is four bytes larger then a jmpbuf, resulting in an
overflow in sigsetjmp. Due to variable reordering this overwrites
the stack cookie.

Signed-off-by: Willem Pinckaers <willem_qemu@lekkertech.net>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Peter: I think I must have missed this one when I converted
       all the jmp_buf to sigjmp_buf in commit 6ab7e546.
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
