Prepare for node 8.0

[bnoordhuis]

Wouldn't it be nice if llnode worked out of the box with node 8? Indeed it would but it won't happen without some even-toed ungulate clipping¹. Here is what needs to be done:

• update tools/gen-postmortem-metadata.py in upstream V8
• back-port changes to nodejs/node
• update llnode to support new object layouts

Bonus points if llnode is taught to print something sensible for WASM and Ignition stack frames/code objects/etc.

¹_{yak shaving}

[bnoordhuis]

Related V8 issue: No build bots build debug-support.cc

[bnoordhuis]

V8 changes: https://codereview.chromium.org/2842843004/