enable sparse endianess checking by default?

[Christoph Hellwig <hch@infradead.org>]

It's been quite a while that we had sparse support in the build system,
and we still disable the checks for endianess annotations by default.

For many subsystems these are arguably the most useful ones, and with
Fengguang's build bot running sparse checks we're also being harder
on driver maintainers to sort these out even where they don't matter
too much for them (e.g. PCI hardware that mostly runs on x86).

Is there any reason to still keep the opt-in for these when running
sparse?

Now running sparse by default if available would be another useful
project, but I'll save that for a later time..