Makefile.extrawarn: turn on missing-prototypes globally

Over the years we went from > 1000 of warnings to under 100 earlier this
year, and I sent patches to address all the ones that I saw with compile
testing randcom configs on arm64, arm and x86 kernels.  This is a really
useful warning, as it catches real bugs when there are mismatched
prototypes.  In particular with kernel control flow integrity enabled,
those are no longer allowed.

I have done extensive testing to ensure that there are no new build errors
or warnings on any configuration of x86, arm and arm64 builds.  I also
made sure that at least both the normal defconfig and an allmodconfig
build is clean for arc, csky, loongarch, m68k, microblaze, openrisc,
parisc, powerpc, riscv, s390, and xtensa, with the respective maintainers
doing most of the patches.

At this point, there are five architectures with a number of known
regressions: alpha, nios2, mips, sh and sparc.  In the previous version of
this patch, I had turned off the missing prototype warnings for the 15
architectures that still had issues, but since there are only five left, I
think we can leave the rest to the maintainers (Cc'd here) as well.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lore.kernel.org/lkml/[email protected]/
Signed-off-by: Arnd Bergmann <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Acked-by: Palmer Dabbelt <[email protected]> # RISC-V
Cc: Richard Henderson <[email protected]>
Cc: Ivan Kokshaysky <[email protected]>
Cc: Matt Turner <[email protected]>
Cc: Dinh Nguyen <[email protected]>
Cc: Thomas Bogendoerfer <[email protected]>
Cc: Yoshinori Sato <[email protected]>
Cc: Rich Felker <[email protected]>
Cc: John Paul Adrian Glaubitz <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: Nicolas Schier <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Richard Weinberger <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Cc: Tudor Ambarus <[email protected]>
Cc: Zhihao Cheng <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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