setjmp() on the sparc uses a sigaltstack(2) system call which is not
implemented causing several interesting programs to drop core.

I tried one based on the info in `libc/arch/sparc/gen/setjmp.s' and <sys/signal.h>
but it is (can not be) complete: the stack location and `onstack' indicator
are taken from the process' user area, but the field `sa_len' is set to zero.

On the other hand, why use sigaltstack(2) in setjmp.s where sigstack(2)
would do just as well?

-pk

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