arm64: traps: Don't print stack or raw PC/LR values in backtraces

Printing raw pointer values in backtraces has potential security
implications and are of questionable value anyway.

This patch follows x86's lead and removes the "Exception stack:" dump
from kernel backtraces, as well as converting PC/LR values to symbols
such as "sysrq_handle_crash+0x20/0x30".

Tested-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>