Just to demonstrate there is really a problem with the gcc installation
(gcc version 3.4.3 (csl-sol210-3_4-branch+sol_rpath)), here is the
linker line:

gcc -m64 -shared
build/temp.solaris-2.10-sun4u-2.5/net/tb0/home/loewis/25/Modules/_struct.o
-L/usr/local/lib -o build/lib.solaris-2.10-sun4u-2.5/_struct.so

and here is what gcc actually invokes (printed with -v):

/usr/sfw/libexec/gcc/sparc-sun-solaris2.10/3.4.3/collect2 -V -G -dy -z
text -Y P,/usr/lib/sparcv9 -Qy -o
build/lib.solaris-2.10-sun4u-2.5/_struct.so
/usr/sfw/lib/gcc/sparc-sun-solaris2.10/3.4.3/sparcv9/crti.o
/usr/ccs/lib/sparcv9/values-Xa.o
/usr/sfw/lib/gcc/sparc-sun-solaris2.10/3.4.3/sparcv9/crtbegin.o
-L/usr/local/lib -L/usr/sfw/lib/gcc/sparc-sun-solaris2.10/3.4.3/sparcv9
-L/usr/ccs/lib/sparcv9
-L/usr/sfw/lib/gcc/sparc-sun-solaris2.10/3.4.3/../../../sparcv9
-L/lib/sparcv9 -L/usr/lib/sparcv9
build/temp.solaris-2.10-sun4u-2.5/net/tb0/home/loewis/25/Modules/_struct.o
-R/usr/sfw/lib -lgcc_s_sparcv9 -R/usr/sfw/lib -lgcc_s_sparcv9
/usr/sfw/lib/gcc/sparc-sun-solaris2.10/3.4.3/sparcv9/crtend.o
/usr/sfw/lib/gcc/sparc-sun-solaris2.10/3.4.3/sparcv9/crtn.o

As you can see, it's picking up -lgcc_s_sparcv9 (from
/usr/sfw/lib/sparcv9/libgcc_s_sparcv9.so), so it does link with the
64-bit libgcc_s.so.1. However, when then trying to import the module, it
complains 

ImportError: ld.so.1: python: fatal: /usr/sfw/lib/libgcc_s.so.1: wrong
ELF class: ELFCLASS32

This is due to the option -R/usr/sfw/lib, which should have been
-R/usr/sfw/lib/sparcv9. Fixing the specs file to change the -R option
actually fixes that problem.

So please don't claim that I did something wrong when there is really a
bug in sfw version of gcc.