Don't build libctf tests in source dir

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.

libctf/testsuite/lib/ctf-lib.exp

@@ -298,7 +298,7 @@ proc run_lookup_test { name } {
 	    }
 
 	    if { $opts(link) == "objects" } {
-		set obj "[file rootname $src].o"
+		set obj "tmpdir/[file rootname [file tail $src]].o"
 		set comp_output [prune_warnings [run_host_cmd "$CC_FOR_TARGET" "$CFLAGS_FOR_TARGET $lookup_flags $src -c -o $obj"]]
 
 		if { $comp_output != ""} {