[foreign-memaccess] RFR 8224037: Remove layout attributes

[Maurizio Cimadamore]

Now that we have alignments, we can finally go ahead and drop layout 
attributes, which is, at present, an overly general mechanism.

It is likely that the need for custom attributes will resurface at some 
point when dealing with foreign function. But this API doesn't need it.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8224037/

Maurizio