Enable -Wstringop-overflow globally

Hi Linus,

Please, pull the following patches that enable -Wstringop-overflow,
globally. These patches have been baking in linux-next for a whole
development cycle.

I waited for the release of -rc1 to run a final build-test on top of
it before sending this pull request. Fortunatelly, after building
358 kernels overnight (basically all supported archs with a wide
variety of configs), no more warnings have surfaced! :)

Thus, we are in a good position to enable this compiler option for
all versions of GCC that support it, with the exception of GCC-11,
which appears to have some issues with this option[1].

[1] https://lore.kernel.org/lkml/b3c99290-40bc-426f-b3d2-1aa903f95c4e@embeddedor.com/

Thanks
--
Gustavo
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