build: fix coverage build

After adding the node_lib target in node.gyp, most of the node source
files are moved to that target. When coverage option is enabled,
corresponding cflags and ldflags are needed in that target as well.
gcovr also needs to check .gcda data for both node and node_lib.

PR-URL: https://github.com/nodejs/node/pull/18409
Fixes: https://github.com/nodejs/node/issues/18402


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>