I have noticed that both dg-do and dg-require-effective-target modify
the value of dg-do-what, which means that dg-do directives must appear
before dg-require-effective-target.

Indeed if the effective-target property is false, but dg-do is
executed later, the test would fail instead of being unsupported.

The attached patch fixes the order on the few testcases where I
noticed it was wrong.

Tested on several arm* and aarch64* targets/multilibs with no regression.

OK?

Christophe.