[testsuite] dg-require-effective-target: skip unneeded checks

[Mike Stump]

On Jun 21, 2011, at 2:02 PM, Janis Johnson wrote:
> This patch causes dg-require-effective-target to return early if the
> test is already being skipped, saving some work.  There's already
> similar code in dg-skip-if.
> 
> OK for trunk, and later for 4.6?

Ok.  Ok for 4.6.  I'm not a huge fan of upvar, but, you didn't create the problem, so, you don't have to fix it.