llvm: Switch to using release tarballs

git checkouts are in excess of 3G, which is not
ideal for everyone to download/clone, instead switch to
fetching release tarball which is ~126M as of 18.1.5 release

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>