ARM: linux-user: Correct size of padding in target_ucontext_v2

The padding in the target_ucontext_v2 is defined by the size of
the target's sigset_t type, not the host's. (This bug only causes
problems when we start using the uc_regspace[] array to expose
VFP registers to userspace signal handlers.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@nokia.com>
