Add sdklibs target to Makefile.

This is the target that the NaCl SDK build will use to build
the libraries it needs.

BUG=None
TEST=make sdklibs

Review URL: https://codereview.chromium.org/11963002

git-svn-id: http://naclports.googlecode.com/svn/trunk/src@658 7dad1e8b-422e-d2af-fbf5-8013b78bd812