MIPS: Use generic ucontext.h

Signed-off-by: Yoichi Yuasa <yuasa@linux-mips.org>
Cc: linux-mips <linux-mips@linux-mips.org>
Patchwork: http://patchwork.linux-mips.org/patch/959/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>