NaCl SDK] nacl_io: remove getcwd() implemenation (rely on glibc/newlib versions)

NaCl SDK] nacl_io: remove getcwd() implemenation (rely on glibc/newlib versions)

The newlib version of getcwd in libnacl now supports
passing NULL of buffer, and recent version of gtest
will gracefully handle getcwd failure in static
constructors so we no longer need our own getcwd
implementation.

This is the second attempt and landing this change.  The
first was reverted:
https://codereview.chromium.org/756333005

R=bradnelson

Committed: https://crrev.com/4fad785312ac90ea3dd6e34926f30adee442ff89
Cr-Commit-Position: refs/heads/master@{#311329}

[bradn, 2015-01-13 19:19:14]

bradnelson@google.com changed reviewers:
+ bradnelson@google.com

[bradn, 2015-01-13 19:19:14]

lgtm

[Sam Clegg, 2015-01-13 20:15:48]

The CQ bit was checked by sbc@chromium.org

[commit-bot: I haz the power, 2015-01-13 20:16:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/851723004/1

[commit-bot: I haz the power, 2015-01-13 21:34:00]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-13 21:35:34]

Patchset 1 (id:??) landed as
https://crrev.com/4fad785312ac90ea3dd6e34926f30adee442ff89
Cr-Commit-Position: refs/heads/master@{#311329}