tzcode-native: Fix compiler setting from 2023d version

tzcode-native: Fix compiler setting from 2023d version

Starting from 2023d version, tzcode makefile does not use anymore "cc"
variable for C compiler, due to Makefile refactoring.
Replacing "cc" with "CC" fixes the issue.

Signed-off-by: Alessio Cascone <alessio.cascone@vimar.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit b3cdfca5ef84ed2054faef9abddef3aeed930e17)
Signed-off-by: Steve Sakoman <steve@sakoman.com>