Add gdb-6.4 sources back, for completeness.

Those come from p9 //toolchain/gdb/gdb-6.4/...@86946
5238 files changed
tree: 9e2b8aca9755c566d527e17d5b89b072f56d9e0e
  1. gdb-6.4/
  2. gdb-6.6/
  3. gdb-7.1.x/