Pass "-cross" arg on linux builds

[sbomer]

Fixes dotnet/runtime#84780. Tested by hooking up the changes locally with runtime's jitformat.py and running python3 ./src/coreclr/scripts/jitformat.py -c /runtime/src/coreclr -o linux -a x64 in our mariner build container.

This might break scenarios where jitutils are used outside of ci on a non-mariner build image. Per discussion with @BruceForstall let's make a quick fix to unblock ci, and if this is problematic we could for example add an extra argument that controls whether the -cross argument is passed to the runtime build.

For context, the -cross argument is necessary after dotnet/runtime#84148 because our linux build images have change from centos 7 to CBL-mariner, and the linux builds are now cross-builds that use a rootfs (even x64 and x64-musl).