Makefile.extrawarn: re-enable -Wformat for clang

Makefile.extrawarn: re-enable -Wformat for clang

There's been an ongoing mission to re-enable the -Wformat warning for
Clang. A previous attempt at enabling the warning showed that there were
many instances of this warning throughout the codebase. The sheer amount
of these warnings really polluted builds and thus -Wno-format was added
to _temporarily_ toggle them off.

After many patches the warning has largely been eradicated for x86,
x86_64, arm, and arm64 on a variety of configs. The time to enable the
warning has never been better as it seems for the first time we are
ahead of them and can now solve them as they appear rather than tackling
from a backlog.

As to the root cause of this large backlog of warnings, Clang seems to
pickup on some more nuanced cases of format warnings caused by implicit
integer conversion as well as default argument promotions from
printf-like functions.

Link: https://github.com/ClangBuiltLinux/linux/issues/378
Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>