Extend the test a little to archives, not that we expect this to
fail. Nor has the lto-18 test ever failed without -flto.
* testsuite/ld-plugin/lto-18b.c (select): Remove.
* testsuite/ld-plugin/lto-18c.c (select): Remove.
* testsuite/ld-plugin/lto.exp: Build archives for lto-18 too,
and run static versions of the test.