fs: improve performance for sync stat() functions

Results from benchmarks:

                                                 improvement confidence      p.value
 fs/bench-statSync.js kind="fstatSync" n=1000000     13.51 %        *** 4.033815e-18
 fs/bench-statSync.js kind="statSync" n=1000000       7.09 %        *** 6.502451e-15
 fs/bench-statSync.js kind="lstatSync" n=1000000      7.43 %        *** 9.979362e-16

CI: https://ci.nodejs.org/job/node-test-pull-request/6568/

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)

• fs