libctf, ld: fix data symbol test with newer GCC

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.

ld/ChangeLog
2021-01-19  Nick Alcock  <nick.alcock@oracle.com>

	* testsuite/ld-ctf/data-func-2.c: Stop removal of the extern foo_t
	symbols by the optimizer.
	* testsuite/ld-ctf/data-func-conflicted.d: Adjust accordingly.