wrapper: resolve /proc/self/cwd only when seen.

This wrapper may run on Mac, which doesn't have /proc/self/cwd
available. This also adds a test to be sure this selective Readlink()
behavior continues to work.

BUG=None
TEST=SDK Tryjob; tests pass

Change-Id: I6f6aaeb7ea45ab8b3d68299422f0594a689ecfcd
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2095763
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Tiancong Wang <tcwang@google.com>
Tested-by: George Burgess <gbiv@chromium.org>
2 files changed