linux-user: use TARGET_SA_ONSTACK in get_sigframe

As reported by Laurent, which should use TARGET_SA_ONSTACK
on arm, microblaze and openrisc targets like we do on all
others. Practical matter is minimal as for almost all archs
SA_ONSTACK is 0x08000000:

http://lxr.free-electrons.com/ident?i=SA_ONSTACK



Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>