Add automatic sysname detection for ARM Linux

Add automatic sysname detection for ARM Linux

Add arm*-linux* to the case statement that attempts to automatically
determine the AFS sysname, similar to the other Linux sysname
determination cases.

Reviewed-on: http://gerrit.openafs.org/334
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit a59df36211d6bc641eeec6d8fd8876a98ce3600a)
Reviewed-on: http://gerrit.openafs.org/462