libgomp: Fix offload_target with additional sources

The offload_target_* effective target checks called libgomp_target_compile in a
way that altered global state if additional sources were configured.

This patch ignores additional sources if the compile check uses no sources (a
compiler probe, only), thus fixing the problem.

libgomp/ChangeLog:

	* testsuite/lib/libgomp.exp (libgomp_target_compile): Don't use
	additional sources if there are no primary sources.

libgomp/testsuite/lib/libgomp.exp

@@ -295,7 +295,9 @@ proc libgomp_target_compile { source dest type options } {
 	set options [concat "$ALWAYS_CFLAGS" $options]
     }
 
-    set options [dg-additional-files-options $options $source $dest $type]
+    if { $source != "" } {
+        set options [dg-additional-files-options $options $source $dest $type]
+    }
 
     set result [target_compile $source $dest $type $options]