build system: re-enable clang use

Change-Id: I6e07fdec449d0b259d77986f65a60aa36d367cc8
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/5747
Tested-by: build bot (Jenkins)