Add support for arm64_linux26

Add support for arm64_linux26

Add support for the arm64/aarch64 architecture on Linux 2.6+. The
param header file is mostly combined from arm and amd64.

Note that the code for syscall interception has not been updated for
arm64, so this will not build on arm64 without support for kernel
keyrings. This also does not define any AFS syscall number, since no
number in the Linux arm64 syscall table is "free" for us to use, as
far as I am aware.

Adapted from initial patches from Micheal Waltz <mwaltz@qualcomm.com>.

Change-Id: I1ee239ded17d8fea3b91b70405215aa1b3f7a6e9
Reviewed-on: https://gerrit.openafs.org/11940
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>