tools: allow passing absolute path of config.gypi in js2c
This PR makes js2c accept "/path/to/config.gypi"
instead of forcing a "config.gypi"
string, which is needed when building Node.js with alternative build systems.
/cc @joyeecheung
Refs https://github.com/nodejs/node/pull/47637 and https://github.com/nodejs/node/pull/46997.