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I have confirmed that gcc 3.4.2 also successfully builds an
ELF 64-bit for 2.5c2 and the interpreter works.

Putting the sparcv9 64-bit shared libraries first in
LD_LIBRARY_PATH also fixes the extension building problem
nnorwitz noted.  Only a few extension modules fail to build
(per the 2.5 Release Candidate 2 news item) because of
dependence of 32-bit ELF class .sos:  

_tkinter (libtk8.4.so, libtcl8.4.so)
_sqlite3 (libsqlite3.so)
_ssl ((libcrypto.a(digest.o))
_hashlib  (libcrypto.a(digest.o))
bz2 (libbz2.a(bzlib.o))

But this might be fixed by recompiling these libraries as
64-bit.

LD_LIBRARY_PATH=/usr/lib/sparcv9:/usr/local/lib/sparcv9: ...

Will report back results attempting 4.0.2 and
LD_LIBRARY_PATH modification to see if extension modules can
be built to mirror 3.4.2 results.