arm: use r12 to pass the hypercall number

arm: use r12 to pass the hypercall number

** This is a guest visible ABI change which requires an updated guest kernel **

Use r12 to pass the hypercall number and r0-r4 for the hypercall
arguments.

Use the ISS to pass an hypervisor specific tag.

Remove passing unused registers to arm_hypercall_table: we don't have 6
arguments hypercalls and we never use 64 bit values as hypercall
arguments, 64 bit values are only contained within structs passed as
arguments.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
[ use #ifndef NDEBUG, fix coding style, expand calling convention comment
  slightly and added a big fat note about ABI change - ijc ]
Committed-by: Ian Campbell <ian.campbell@citrix.com>