gcc_release (build_tarfile): Build .bz2 files instead of .gz files.

	* gcc_release (build_tarfile): Build .bz2 files instead of .gz files.
	(build_bz2): Rename to build_gzip and create .gz files from .bz2
	files instead of the other way around.
	(build_gzip): New function.
	(build_diffs): Build .bz2 files instead of .gz files.
	(build_diff): Use .bz2 files instead of .gz files.
	Make an error message independent of the compression format.
	(MODE_BZIP2): Rename to MODE_GZIP.  Related changes to the
	invocation of this script.
	(MODE_GZIP): New variable.
	(OLD_TARS): Use .bz2 archive of the previous snapshot.

From-SVN: r68074