linux-unwind.h (alpha_fallback_frame_state): Cast 'mcontext_t *' &rt_->uc.uc_mcontext to 'struct sigcontext *'.

	* config/alpha/linux-unwind.h (alpha_fallback_frame_state):
	Cast 'mcontext_t *' &rt_->uc.uc_mcontext to 'struct sigcontext *'.

From-SVN: r269053