Fix NACL_SDK_LIB variable in i686 builds.

The glibc shared objects live in x86_64-nacl for both i686
and x86_64 builds (distinguished by lib32 and lib64).  They
were being incorrectly set to i686-nacl which was casing sel_ldr
tests to fail:
http://build.chromium.org/p/client.nacl.ports/builders/linux-glibc-0/builds/190/steps/i686%20nacl-mounts/logs/stdio

BUG=

Review URL: https://codereview.chromium.org/11876035

git-svn-id: http://naclports.googlecode.com/svn/trunk/src@656 7dad1e8b-422e-d2af-fbf5-8013b78bd812