cxgbe(4): Add a knob to enable/disable PCIe relaxed ordering. Disable it by
default when running on Intel CPUs.

This is a crude fix for the performance issues alluded to in these Linux commits:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=87e09cdec4dae08acdb4aa49beb793c19d73e73e
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a99b646afa8a02571ea298bedca6592d818229cd

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Sponsored by: Chelsio Communications