Fix 0.9.8 FIPS capable OpenSSL build.

The object file bn_lib.o is excluded from FIPS builds which causes
a linker error for BN_consttime_swap. So move definition from bn_lib.c
to bn_gf2m.c

This change is *only* needed for OpenSSL 0.9.8 which uses the 1.2
FIPS module.