arm64: Don't call dump_mem() on stack memory during __die()

With multiple CPUs online, these memory dumps may consume a very
large amount of log buffer space, pushing out useful information.

Drop the call to dump_mem() for stack memory. The actual string-
formatted call stack will still be printed, which should be sufficient
for identifying the context of a panic. Stack memory can be collected
from full memory ramdumps if needed.

Change-Id: I2d08b49217b3838d3d3e7f8deceb937bb90ffaf7
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>