[NaCl SDK] Set PEPPER_DIR properly when building naclports.

This should be "pepper_NN", but was coming through as a path.

BUG=none
R=sbc@chromium.org

Review URL: https://codereview.chromium.org/62753002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233322 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233322