kbuild: enable -Wformat-truncation on clang

This warning option still produces output on gcc but is now clean when
building with clang, so enable it conditionally on the compiler for now.

As far as I can tell, the remaining warnings with gcc are the result of
analysing the code more deeply across inlining, while clang only does this
within a function.

Link: https://lore.kernel.org/lkml/20240326230511.GA2796782@dev-arch.thelio-3990X/
Link: https://lore.kernel.org/linux-patches/20231002-disable-wformat-truncation-overflow-non-kprintf-v1-1-35179205c8d9@kernel.org/
Link: https://lkml.kernel.org/r/20240415122037.1983124-5-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nicolas Schier <nicolas@fjasle.eu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>