arch: Get the byte order from sys and not TheISA::.
arch: Get the byte order from sys and not TheISA::.

This is a small step which localizes the use of TheISA, hopefully making
it easier to eliminate in the future.

Change-Id: I13472ed69e12a3c753e2dea91b9c7ca813bfc0e7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32919
Reviewed-by: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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