Fix dejagnu test support for --tool_opts

Right now it concatenates it with the existing options and then appends
it to that list, fix it to simply append it as is, same as it is done
with the other variables.

Tested by running the following command which includes gcc options:
  $ make check RUNTESTFLAGS="--tool_opts '-Werror'"

Without this patch, all the tests fail. With it, the test succeed.
Inspecting the logs shows that -Werror was indeed used when compiling
the test sources.

testsuite/lib/libffi.exp

@@ -182,7 +182,7 @@ proc libffi_target_compile { source dest type options } {
     # TOOL_OPTIONS must come first, so that it doesn't override testcase
     # specific options.
     if [info exists TOOL_OPTIONS] {
-	lappend  options [concat "additional_flags=$TOOL_OPTIONS" $options];
+	lappend  options "additional_flags=$TOOL_OPTIONS"
     }
 
     # search for ffi_mips.h in srcdir, too