Running the libctf testsuite currently leaves big-struct-ctf.o in
libctf/testsuite/libctf-lookup/.
It ought to be possible to make the binutils source dir read-only,
and putting compiler output in the source dir leads to interesting
effects when testing multiple binutils targets in parallel.
* lib/ctf-lib.exp (run_lookup_test): For "link: objects" compile
objects in tmpdir.