Fix broken flags

I got a build on Debian x86_64 with gcc-4.7.2. To do so, I had to play with some flags in tools/waf/bde/default.opts.

  -!! *-*-x86_64-*-gcc     _   CC64FLAGS   = -m32 -march=pentium2 -mtune=opteron
  -!! *-*-x86_64-*-gcc     64  CC64FLAGS   = -m64 -mtune=opteron
  +!! *-*-x86_64-*-gcc     _   CC64FLAGS   = -m64
  +!! *-*-x86_64-*-gcc     64  CC64FLAGS   = -m64