alpha: add special umount2 handling
Only on Linux alpha __NR_oldumount is defined and a umount not
umount2 syscall, but with two parameter is used.
Add special handling for it and an alias for umount2() users.
There was a discussion about this special handling, but it seems
it was never committed upstream:
http://marc.info/?l=linux-alpha&m=137455037930738&w=2
Runtime tested with qemu-alpha and a statically linked busybox
binary.