patch by Mahesh Bodapati and Jaydeep Patil of Imagination
Technologies.

This adds complete aarch64 target support including bigendian subarch.

Some of the long double math functions are known to be broken otherwise
interfaces should be fully functional, but at this point consider this
port experimental.

Initial work on this port was done by Sireesh Tripurari and Kevin Bortis.

if these are to be supported, they belong in the main stat.h, not
repeated for each arch.

thanks to Peter Mazinger (psm) for pointing many of these issues out
and submitting a patch on which this commit is loosely based