os: improve os.tmpdir() method

Improve os.tmpdir() speed, avoid duplicate REGExp test.

following is benchmark result:

Before:

$ node benchmark/os/tmpdir.js os/tmpdir.js n=100: 1720.48425 os/tmpdir.js n=1000: 2028.35615 os/tmpdir.js n=10000: 1984.48570

After: $ ./node benchmark/os/tmpdir.js os/tmpdir.js n=100: 15024.67502 os/tmpdir.js n=1000: 121723.59639 os/tmpdir.js n=10000: 129720.92049