os: improve loadavg() performance

Results with included benchmark:

                         improvement confidence      p.value
 os/loadavg.js n=5000000    150.91 %        *** 1.380968e-49

CI: https://ci.nodejs.org/job/node-test-pull-request/6563/

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

• os