0.8.19.26: fix bignum printing:

0.8.19.26: fix bignum printing:
            * algorithm by Harald Hanche-Olsen.
            * good: less memory-intensive, faster for smallish
               bignums.
            * bad: more CPU-intensive for large eg. 2^1000000)
               bignums, around order of 2^10000000 becoming essentially
               unprintable.

           lazy alien resolution improvement:
            * resolve undefined functions to the address of
               undefined_alien_function instead of the guard
               page to work around the bus error on ppc/darwin.
            * also buys us discrimination between undefined alien
               variables and functions (on all platforms).

           note ppc/darwin problems revealed by tests/ in BUGS, and disable
           those tests:
            * floating-point overflow not signalled
            * type-error from sb-profile:report
            * bit-and with huge bit-vectors fails with a lost
               gc-invariant (maybe a general cheney-gc problem?)

            => All tests now pass on ppc/darwin, note this in the message
               at the end of make.sh, marking x86/Linux and ppc/Darwin
               as platforms where all tests are supposed to pass.