[foreign-memaccess] RFR 8223786: Rename layout annotation to layout attribute

[Maurizio Cimadamore]

The term 'annotation' is too polluted when it comes to Java. Replacing 
it with something more neutral like 'attribute' could work better:

http://cr.openjdk.java.net/~mcimadamore/panama/8223786/

This is a good first step, but longer term I'm also considering dropping 
annotations and have layout names, size and alignment to be modeled more 
directly in the API. After all, the only reason why we need layout 
attributes at all in this API is to express alignment constraints; in 
the next few days I'll put together a prototype which implements 
alignments checks - and see whether it's feasible to expose alignment as 
a first class property of layouts (as sizes are).

Maurizio