tests: default to 'cc' if CC not set

By default FreeBSD does not have 'gcc' in the PATH (on common platforms).
As on Linux 'cc' is available as a link to the default compiler (Clang or
GCC), so just use 'cc'.

Signed-off-by: Ed Maste <emaste@freebsd.org>
Message-Id: <20191115155108.39488-1-emaste@freefall.freebsd.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>