kern: When dumping dmesg, detect the byte order dynamically.

kern: When dumping dmesg, detect the byte order dynamically.

The dmesg dumper has access to the system object and so has access to
the getGuestByteOrder accessor. Use that instead of TheISA to determine
the byte order.

Change-Id: I4df7b1bcd807aaced1d7dc8d2030123e2d4d1d2b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22365
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>