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: nodejs#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>

node.gypi

@@ -325,7 +325,8 @@
                    '-O0' ],
        'cflags': [ '--coverage',
                    '-g',
-                   '-O0' ]
+                   '-O0' ],
+       'cflags!': [ '-O3' ]
     }],
     [ 'OS=="sunos"', {
       'ldflags': [ '-Wl,-M,/usr/lib/ld/map.noexstk' ],