Don't include undef in cross reference table. From upstream's 55f7fb8848b6e4bec291724a479e1580d6f407d6. Change-Id: I54ebc182addbf643bebc78aab03ba1327e24e2e7 Reviewed-on: https://boringssl-review.googlesource.com/2470 Reviewed-by: Adam Langley <agl@google.com>