arm64: process: dump memory around registers when displaying regs

A port of 8608d7c4 to ARM64.  Both the
original code and this port are limited to dumping kernel addresses, so
don't bother if the registers are from a userspace process.

Change-Id: Idc76804c54efaaeb70311cbb500c54db6dac4525
Signed-off-by: Greg Hackmann <ghackmann@google.com>