Rename the 'default' part number in clang cross-file to fix the
following issue:
config/arm/meson.build:238:2: ERROR: Problem encountered: Unsupported
part number default of implementer generic. Please add support for it or
use the generic (-Dmachine=generic) build.
Fixes: 3d01d65ba0d3 ("config: add aarch64 clang for Ubuntu 18.04")
Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>
7390f02315