Using numbers is sensitive to moving things around. Instead, use the
names and enforce, for sections, that they are unique. Names would be
enforced too, but there's a table-of-contents bug around #ifdefs to
resolve first.

Change-Id: I8822e8ba8da9ed3ee4984365b8a64932d16d5baf
Reviewed-on: https://boringssl-review.googlesource.com/5826
Reviewed-by: Adam Langley <agl@google.com>