[Metrics] split 'undef functions' into 'spec-only' and 'unspec and undef'

src/plugins/metrics/metrics_cilast.ml

src/plugins/metrics/metrics_cilast.mli

tests/libc/oracle/fc_libc.0.res.oracle

tests/metrics/oracle/cyclo_comp5.res.oracle

tests/metrics/oracle/func_ptr.0.res.oracle

tests/metrics/oracle/func_ptr.1.res.oracle

tests/metrics/oracle/libc.0.res.oracle

tests/metrics/oracle/libc.1.res.oracle

tests/metrics/oracle/reach.res.oracle

tests/metrics/oracle/unreachable.res.oracle

tests/metrics/oracle/variadic-stdlib-generated.res.oracle