Create a new cache for the --enable-debug-tcg builds which is separate
from the normal debug builds which generate different code. We also
enable debug-tcg for the new plugins based builds as we want to ensure
any breakage to TCG is picked up by the sanity checks.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>