cmake/toolchain: Support LLVM source profiling/coverage for native boards

Add a a new source coverage for native builds
and new kconfig choice of COVERAGE mode to select which:
* COVERAGE_NATIVE_GCOV: what we had until now with native builds
* COVERAGE_NATIVE_SOURCE: a new LLVM source coverage mode
* COVERAGE_GCOV: the old COVERAGE_GCOV (embedded gcov data generation).

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>