Kbuild: enable -fms-extensions

Kbuild: enable -fms-extensions

Once in a while, it turns out that enabling -fms-extensions could
allow some slightly prettier code. But every time it has come up, the
code that had to be used instead has been deemed "not too awful" and
not worth introducing another compiler flag for.

That's probably true for each individual case, but then it's somewhat
of a chicken/egg situation.

If we just "bite the bullet" as Linus says and enable it once and for
all, it is available whenever a use case turns up, and no individual
case has to justify it.

A lore.kernel.org search provides these examples:

- https://lore.kernel.org/lkml/200706301813.58435.agruen@suse.de/
- https://lore.kernel.org/lkml/20180419152817.GD25406@bombadil.infradead.org/
- https://lore.kernel.org/lkml/170622208395.21664.2510213291504081000@noble.neil.brown.name/
- https://lore.kernel.org/lkml/87h6475w9q.fsf@prevas.dk/
- https://lore.kernel.org/lkml/CAHk-=wjeZwww6Zswn6F_iZTpUihTSNKYppLqj36iQDDhfntuEw@mail.gmail.com/

Undoubtedly, there are more places in the code where this could also
be used but where -fms-extensions just didn't come up in any
discussion.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: David Sterba <dsterba@suse.com>
Link: https://patch.msgid.link/20251020142228.1819871-2-linux@rasmusvillemoes.dk
[nathan: Move disabled clang warning to scripts/Makefile.extrawarn and
         adjust comment]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>