build: embed the mdb_v8.so into the binary

This builds and includes the mdb_v8.so in the binary of node so mdb can be sure to always use the latest version

This helps make sure that v8.so that mdb uses is in sync with the version of node that generated the core file.

Also a small utility `tools/mdb/extract_dmod.c` which can be used to extract `v8.so` from a core file if you're on a platform with an older mdb or an out of date v8.so