zlib: use .gz archive and set a PREMIRROR

zlib: use .gz archive and set a PREMIRROR

When a new zlib release is made, the top-level URL is no longer available
and it is only available as a .gz under the /fossils/ directory.

When this happens the source fetch fails and bitbake noisily warns that
it is using the mirrors.  Avoid this by using the .gz tarball and add
the /fossils/ directory to PREMIRRORS so fetches will check there too.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>