Add static page size for aarch64

If this page size isn't set statically, the page size is instead loaded
from libc which calculates it when the process is initialised. If muslc
is not providing the runtime, the PAGE_SIZE becomes 0.

This can lead to a strange issue where malloc will assume that every
call to sbrk fails (including the first) and will always fall back to
mmap. For some reason, this ends up confusing malloc after an initial
allocation of 0 bytes beginning at the _end_ of the static morecore
region.

add aarch64 port

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.