Create a dedicated source file to share among platforms on which we
implement hrtime using POSIX `clock_gettime`. De-duplicate our
existing copies of this implementation on such platforms.
PR-URL: https://github.com/libuv/libuv/pull/1239
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
