xen: arm: define 64-bit guest hypercall calling convention.

xen: arm: define 64-bit guest hypercall calling convention.

As well as using x<N> rather than r<N> registers for passing arguments/results
as mandate the use of x16 as the hypercall number.

Add some pedantry about struct alignment layout referencing the ARM Procedure
Calling Standard to avoid confusion with the previous "OABI" convention. While
at it also mandate that hypercall argument structs are always little endian.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>