Changed URLs of GCC tarballs.

* compilers.jam: Followed the move and rename of the file from
                 `gcc-release-versions.sh` to `gcc/versions.sh`.
* gcc-release-versions.sh -> gcc/versions.sh:
  * Moved and renamed.
  * Removed the URL of `ftp.dti.ad.jp` because it is no longer reachable.
  * Added a URL of `ftp.kddilabs.jp`.
* gcc/jamfile:
  * Removed the URL of `ftp.dti.ad.jp` because it is no longer reachable.
  * Added a URL of `ftp.kddilabs.jp`.