Mk/Uses/cmake.mk: Define -DNDEBUG when BUILD_TYPE is set to RELEASE

CMake defines -DNDEBUG when BUILD_TYPE is set to RELEASE by default
however default flags gets dropped when CMAKE_C_FLAGS_RELEASE and
CMAKE_CXX_FLAGS_RELEASE are (re)defined. This causes some projects
to build with debugging by default which needs to be disabled
by hand if they do not set -DNDEBUG manually either in ports
Makefile and/or in CMake's Makefile. To follow upstream add
-DNDEBUG to our RELEASE flags.

PR: 266721
Reviewed by: arrowd
Approved by: portmgr (tcberner)
Differential Revision: https://reviews.freebsd.org/D36695
Exp-run by: antoine