gcc_release (announce_snapshot): Use ${RELEASE} instead of ${BRANCH}-${DATE} to refer to the snapshot directory.

	* gcc_release (announce_snapshot): Use ${RELEASE} instead
	of ${BRANCH}-${DATE} to refer to the snapshot directory.
	Put README file into the snapshot directory.

	(RELEASE): Define as ${BRANCH}-${DATE} for snapshots.
	(FTP_PATH): Use ${RELEASE} in case of snapshots.
	(TAG): Ditto.
	(OLD_TARS): Adjust to new directory scheme for snapshots.

From-SVN: r69584