kbuild: move -Werror from KBUILD_CFLAGS to KBUILD_CPPFLAGS

CONFIG_WERROR turns warnings into errors, which happens only for *.c
files because -Werror is added to KBUILD_CFLAGS.

Adding it to KBUILD_CPPFLAGS makes more sense because preprocessors
understand the -Werror option.

For example, you can put a #warning directive in any preprocessed code.

    warning: #warning "this is a warning message" [-Wcpp]

If -Werror is added, it is promoted to an error.

    error: #warning "this is a warning message" [-Werror=cpp]

This commit moves -Werror to KBUILD_CPPFLAGS so it works in the same way
for *.c, *.S, *.lds.S or whatever needs preprocessing.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>