Use the proper C++ compiler to run C++ tests

Running the C compiler with -shared-libgcc -lstdc++ does not work on
non-GCC compilers.

testsuite/lib/libffi.exp

@@ -222,6 +222,10 @@ proc libffi_target_compile { source dest type options } {
 	lappend options "libs= -lpthread"
     }
 
+    if { [string match "*.cc" $source] } {
+	lappend options "c++"
+    }
+
     verbose "options: $options"
     return [target_compile $source $dest $type $options]
 }