[PATCH 1/3 trunk] libctf, ld: fix data symbol test with newer GCC

[Nick Alcock]

On 18 Jan 2021, Nick Alcock via Binutils uttered the following:

> GCC 11+ spots that the extern var_1 and var_666 declarations in this
> test are unused, and removes them, thus stopping them from appearing as
> conflicted data symbols and rendering the test pointless.  Use them in a
> function unique to this TU to prevent them from being eliminated.

This, and all other (more significant!) commits in this series, are now
pushed to master, but not yet to the release branch until Nick acks my
doing that.