[foreign-memaccess] RFR 8228447, 8228444

[Maurizio Cimadamore]

Hi,
when addressing review comments, I decided to fold both patches into 
one, to facilitate the review (after all both patches are touching the 
same files).

http://cr.openjdk.java.net/~mcimadamore/panama/8228447%2b8228444_v2/

Changes:

* BYTE_8 -> BITS_64, etc.
* added constants for padding layouts - e.g. PAD_8, PAD_16, ...
* Changes all tests to use constant layouts where possible (e.g. where 
both size and endianness are static constants)

Maurizio