The new-style struct sigcontext has a member __sc_mask13 of type
sigset13_t, but the typedef of sigset13_t in sys/signal.h is
protected by an #ifdef.

On a mips, that breaks all userland code which includes signal.h.  I
think the i386 has the same problem.  The alpha signal.h dlecares
__sc_mask13 as a long, so it's OK.

I think we need to either expose sigset13_t in sys/signal.h, or change
all the MD headers to use a fundamental type of the correct size for
__sc_mask13.