arm64/crashdump-arm64: deduce the paddr of _text

Since kernel commit e2a073dde921 ("arm64: omit [_text, _stext) from
permanent kernel mapping"), the physical address of 'Kernel code' in
/proc/iomem is mapped from _text, instead, from _stext.

Taking the compatibility into account, it had better deduce the paddr of
_text despite of the unavailability through /proc/iomem. It can be
achieved by utilizing the fact _text aligned on 2MB.

Signed-off-by: Pingfan Liu <piliu@redhat.com>
Cc: Simon Horman <horms@verge.net.au>
To: kexec@lists.infradead.org
Signed-off-by: Simon Horman <horms@verge.net.au>