xen/arm64: fix stack dump in show_trace

xen/arm64: fix stack dump in show_trace

On aarch64 the frame pointer points to the next frame pointer and the return
address is the previous stack slot (so below on the downward growing stack,
therefore above in memory):

       |<RETURN ADDR>      ^addresses grow up
 FP -> |<NEXT FP>          |
       |                   |
       v                   |
       stack grows down.

This is contrary to aarch32 where the frame pointer points to the return
address and the next frame pointer is the next stack slot (so above on the
downward growing stack, below in memory):

 FP -> |<RETURN ADDR>       ^addresses grow up
       |<NEXT FP>           |
       |                    |
       v                    |
       stack grows down.

In addition print out LR as part of the trace, since it may contain the
penultimate return address e.g. if the ultimate function is a leaf function.

Lastly nuke some unnecessary braces.

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