package/binutils: remove support for 2.27

Now that binutils 2.30 has been introduced, and we have moved to
2.29.1 as the default version, it is time to drop support for binutils
2.27.

Also use xz-compressed tarball for all official supported version.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>