kern: Stop using TheISA::GuestByteOrder in Linux::dumpDmesg.

kern: Stop using TheISA::GuestByteOrder in Linux::dumpDmesg.

This value is already read from the system object in that same function.
We should use that instead of getting the value ourselves.

Change-Id: I0a442cd4892f50ad0179884bebf3eb52881c022f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32924
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>