Initial version of AArch64 support.
Initial version of AArch64 support.

Adds AArch64 knowledge to:
* configure,
* make files,
* sources,
* tests, and
* documentation.

ebfull/rand,bluss/rand,retep998/rand,arthurprs/rand,shepmaster/rand,GrahamDennis/rand,achanda/rand,huonw/rand,bhickey/rand,waynenilsen/rand