Don't modify the PWD in gyp_v8

When running gyp in parallel mode (soon the default), we need to be able
to find the script by looking at the path relative to the PWD when the
script was originally invoked.

Instead of changing the directory and relying on relative paths, just
use absolute paths (as gyp_chromium does).

BUG=v8:2973
R=ulan@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/122183004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18439 ce2b1a6d-e550-0410-aec6-3dcde31c8c00