build: always use V8_ENABLE_CHECKS in debug mode

Define `V8_ENABLE_CHECKS` in `common.gypi` for the debug mode.
Without this, these checks would only be present in the object files
generated from the V8 build, and so for inline functions in v8.h
multiple different definitions could be generated, where one definition
includes the check and the other does not.

Refs: https://github.com/nodejs/node/pull/11975#discussion_r108005423
PR-URL: https://github.com/nodejs/node/pull/12029


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jason Ginchereau <jasongin@microsoft.com>
Reviewed-By: Hitesh Kanwathirtha <hiteshk@microsoft.com>