what do to if there is no %nil

[Mike Gran]

Hi-

I tried building master with the --disable-elisp configure option.
When that option is enabled, libguile no longer contains the %nil value.
The assembler expects %nil to always exist.

I made a quick and dirty modification in 
7beae9f15ac49f144a15d3ebfe80c2f29e43b3ff just to get it to build; but,
I'm sure that a prettier solution should implemented.

Thanks,
-Mike