Last week we agreed to clarify that attribute aligned on a function
can decrease its alignment if it hasn't been previously declared
with one.  Attached is this change.

Besides the above, I also mention that the attribute specifies
the alignment of the first instruction of the function (in case
that wasn't obvious) , and that it has no effect in the absence
of a definition.

I wrote some tests to convince myself this actually works as
I expected.  Where it does I add those tests to the testsuite
via this patch.  Where it doesn't I opened a bug (PR 88345).

Martin