Write something to LOG to indicate the value of DEBUG_LEVEL that was used to compile RocksDB

When I am debugging based on LOG files I want to know whether it was created by a build for which debugging and assertions were enabled. I don't think that is done today. If I run db_bench with DEBUG_LEVEL=1 builds I get this:
```
WARNING: Assertions are enabled; benchmarks unnecessarily slow
```

So I know RocksDB can put something similar in LOG.

I do see some DEBUG messages later in LOG, like this, but I want something at the start of the LOG file.
```
2023/11/30-00:56:33.157351 3958909 [DEBUG] [/db_impl/db_impl_files.cc:374] [JOB 1] Delete /tmp/rocksdbtest-1000/dbbench/MANIFEST-000001 type=3 #1 -- OK
```

Start by running db_bench with a DEBUG_LEVEL=1 build and inspect the existing assertion warning and the beginning of the generated LOG file. Trace where the LOG is initialized and where the build's DEBUG_LEVEL can be identified. Done means the LOG starts with a clear indication of the DEBUG_LEVEL used to compile RocksDB, including when debugging and assertions are enabled.

Written by the indexing model from the issue text.