[gcc] Correct configure check so that it matches i686-pc-nacl

My earlier change was accidentally too strict, and did not match the
target configuration when it had been expanded from the argument in
"--target=nacl" to "i686-pc-nacl".

BUG=http://code.google.com/p/nativeclient/issues/detail?id=660
TEST=build full-gcc against nacl-glibc

Review URL: http://codereview.chromium.org/3190016