xen: arm: tweak arm64 stack frame layout

xen: arm: tweak arm64 stack frame layout

Correct definition of UREGS_kernel_sizeof and use it.

Correct adjustment of stack on entry and exit.

Add 64-bit versions of the build time checks for stack pointer alignment
correctness when pushing the stack frames.

Lastly, correct the padding in the stack frames to properly align the inner and
outer frames and also avoid an unnecessary 64bit padding field.

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