Testsuite patch to make cxa_atexit test available in gcc subdir

[Benjamin Kosnik]

> Actually, only dg-require-cxa-atexit calls check_v3_target_cxa_atexit.
> The tests all contain calls to "dg-require-cxa-atexit", not
> to "check_v3_target_cxa_atexit" so no tests need to be changed.

Even better.

> I will check it in with this name change and with the ChangeLog comment
> change.

Sounds great, thanks.

-benjamin