build: disable -O3 for C++ coverage

The `cflags` for `--coverage` included `-O0` so far, but that was
overridden by a later `-O3`. Resolve that by adding
`'cflags!': [ '-O3' ]` and increase coverage accuracy.

Ref: https://coverage.nodejs.org/
PR-URL: https://github.com/nodejs/node/pull/12406


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>